E-Reader recommendations

I use a Kobo, sideloaded with KoReader. I use Calibre to manage my ebooks, which takes a bit of a learning curve, but is definitely worth learning!

Kobo for sure. Great devices.
You bought the kindle books, Amazon took away what you bought. Anna’s archive without any guilt - boom, your kindle books are now yours to read on any device of your choosing again.
You don’t need to fiddle with calibre or managing files. This website works great for transferring books to the kobo. https://send.djazz.se/
You’re welcome :)

The Kobo is great. Very moddable, you can set it up in sideload mode so you never even need to register for an account, just move files to it via USB.

I've heard Kobo makes good ereaders and they have cool stuff like library integration. I'm very happy with my Kindle Oasis (because I largely ignore the Amazon ecosystem 🏴‍☠️), but if I had to replace it I would probably look at Kobo.

i have a kobo which i adore. It's a great little device. Pretty simple to sideload if that's your thing. Just plug in, drag and drop to the root folder of the kobo, then eject drive and unplug. it auto imports them and stuff. The draw back though, kobo plus doesn't have as many books as kindle unlimited, if that's your thing. but all in all, kobos are great.
As for transferring books from kindle to kobo, i don't believe you can anymore. amazon closed that process down, and now I believe you can't do it. Which sucks balls.

I have a boox for bigger screen reading and a Hisense touch for on the go. I like them both. Wish I could more easily tweak the boox, and wish I could more easily ditch all the vendor crapware off the Hisense, but they do their jobs well enough. Hisense gets bonus points for being a hi-fi device that can drive my IEMs.

Kobo Libra 2 checking in. The ability to directly load epub files (and other formats) directly onto the drive and bypass the store is a game changer.
- Not defending Amazon, but you can do this with Kindle too. I've had my kindles in airplane mode since I bought them (for me and my wife) and I'm adding books directly with Calibre anytime I want. You don't need their whole "ecosystem".
- I also have a Libra 2 and it's great! Nice size screen and very lightweight. Just to note as the OP asked for colour, the Libra 2 is B&W but they now do Libra and Clara colour versions :)

I have a Kobo Libra 2 and can sync it wirelessly to my Calibre library. Love it!
- How do you sync it wirelessly? The most common solution I found online for wireless book transfer was to use the web browser on the kobo and download books from the calibre content server
  
  I use Calibre Web for this.
  
  Install koreader or Pluto (or just install both)
  enable calibres Server Funktionality
  connect to Server with plugin

Kobo is the way. Open system. Not locked like kindle. You can "jailbreak" it in 5 seconds. Linux based. They send you firmware updates even when it's "jailbroken". You can bring your own books and they won't fuck with your book covers like kindle does (I would use calibre to convert your books to kepub to get all the features on a book). No ads on the lock screen. I love my kobo Sage. I'm waiting for them to release a 10" one with high PPI so I can buy it. In all honesty, even the android ones are better than kindle. Basically, almost all non-kindles are good, except for a few random Chinese brands that have their own weird OS.
- I read some tests about Kobo. Nice to know people are satisfied. Definitely will look at it again.

I have an older kobo, the h20 Libra, some years old. I have my eye on a new version but I don't replace things that aren't broke. Let me tell you, that Libra will last me years to come still, amazing quality. I haven't even mentioned the easy USB connect to add books, or the great Kobo store.
My next one will definitely be a Kobo as well.
- I have a kobo from 2016 that still works alright. I kind of want to buy a newer model but it's not broken yet so...
  
  Terrible, such sturdiness. I'm starting to fear this will be my last e-reader ever... :)

I'd recommend a Kobo Clara. They do do a colour variant but the next part of my recommendation might not be able to cope with it, I don't know.
That next part is KOReader - you can install it 'over the top' of the standard Clara software so it becomes the working environment. Its got so many pluses I don't have time to list them all but it plays nice with Calibre, you can SSH to it, FTP to it, even access it from some cloud services. I can't say if it supports colour or not though as I don't use a colour Clara.
Transferring your current collection is probably a non-starter due to the recent limitations Amazon have put on downloading files. Getting new ebooks is up to you, there's loads of both legal and not so legal places to source books from, depending on how you feel about DRM.
In terms of transferring books - as well as the ways I mentioned above it also supports OPDS and of course standard USB cabled transfers too.

I've researched this and I'll be buying a Kobo once my Kindle finally dies (it's lived a hard life). I've bought DRMed books from some non-Amazon sources and had to go through the steps to strip their DRM so I could read them on the Kindle - this was using Calibre. Mostly those other sources were using Adobe Digital Editions (DeDRM can handle it).
I have yet to find if there's a self-hosted option that would replicate what Amazon does - i.e. tracks read position and lets you download and read via an Android app or a website. I do have Calibre-web set up, but haven't fully looked into what it can do yet.
- Look into koreader. They host for you without an account. All you need is a username. You can read the same book and track your progress on different devices through koreader. If that's what you mean

I got a deal on the Kobo Libra Colour recently, and I'm very happy with it as a replacement for my old Kindle. The way Amazon walls you in made sure it had to be something other than Kindle.

I got a Boox about a year ago and am very happy with it. In addition to the built in reader, I have the Kindle, B&N, Kobo and Google Play books apps on it. I only buy epubs without DRM these days, and I've de-DRMed all my purchases from all these platforms and backed them up, but if there's something I want to re-read, I find it's easier to use the apps.
I could put other apps on it, since it's essentially an Android device, but I keep it strictly for reading.

I have a Vivlio, a French ebook reader, and I'm quite happy with it. I have a Kindle Paperwhite too for Amazon purchased books, but mainly use the Vivlio. I'm not sure if it's available in the States, or even outside of France.

Im running a Kobo Libra Colour before that an old model Kindle that crapped itself.
Lots to like
Things I don't like
No charging LED so you know when it's fully charged
My books are sidleloaded from Calibre and 70% of the time when i go to start reading a book that I'm part way through, it opens on a different page , often 20-40 pages away, so much confusion. Not sure if it also does this to books sourced from the Kobo bookstore, or its a Calibre thing
Used to piss me off the power button being on the back, flip the cover over to read, it covers the power button. Put the Libra down, come back to it, the kobo cover covered the power switch, DOH! . They have since allowed a work around in the firmware where you can press the page turn button to restart reading, so it's no longer really an issue.

A lot of people will recommend Kobo readers, but also look into Pocketbook readers. I have their Basic Lux, which is a barebones eReader with a black and white screen. Even though it's lower screen size and resolution, I use it for manga and I think it looks great. They have color ereaders, and some that even run on android (but that takes away from battery life).
- Seconding Pocketbook. I'm using the older Touch3HD, which works just wonderfully and comes 100% without any DRM BS.
  They do have their own store but I haven't used it yet.
  https://pocketbook.ch/en-ch/catalog

I think Kobo is the best choice, I use it since 10 years with Calibre.

I have used PocketBook touch lux 5 but the screen cracked twice. (User error but still, screen is a weak link)
KoReader made it so snapy I couldn't believe it at first.
Now I have got kindle as I received it as a gift, so I jailbroke it and also installed KoReader on it.

Onyx Boox are the best imo. I am not sure exactlyabout which the latest model is, mine is a few years old.
Android, has a web browser, file transfer over network. Getting books is very easy and you can even put a library app on there, or any store that is on an app store. But also of course just download from the browser is fine. You should be able to strip drm from current purchases with calibre and then just use the network file transfer and get them on there, for example. There is support for any mayor format, epub, mobi, pdf, cbz..
Battery lasts very long time. Has backlight. Newer models have color but I have not tried those myself.
The reader is awesome and has built in dictionary and translation for individual words. Some basic reading tracking statistics as well. You can of course install any other reader you want from app store or apk.
- I don't agree with this. Boox devices are notorious for not getting updates. As they are running on Android, security updates are crucial. But they keep churning out so many devices per year that they usually don't bother to keep existing ones up to date.
  In general, I wouldn't recommend an Android device for reading books. They are usually heavier than regular readers like Kobos for example. They also have atrocious battery life. Compared to a regular tablet it might be fine but compared to a dedicated ereader it's usually crap.
  Their UI is also not as intuitive and needs some getting used to. But I guess that's subjective.
  I think for only reading there is no need to get a full fledged Android device.
  Lastly, there is also the fact that it's a Chinese company. People can make of that what they want. Kobo is Canadian, although the parent company, Rakuten, is Japanese.
  
  You'd figure not providing the source code of GPL licensed applications they use should free up plenty of time to make updates.
  
  Battery for a week is pretty good to me. The UI is fine, but as you say that is personal preference. I agree that it is worth mentioning that it is chinese. It is not heavy at all, it is very light.
  Android is useful if you don't want to be tied to a specific store, for being able to read documentation and articles as well as easily getting new books. It is not meant to be used for web browsing or any serious app use, but I disagree that having Android is pointless, as it allows for alternate readers (like Mihon) and getting books from the web.

If you are looking at the onyx boox leaf range, I think a new one is being announced in the next week or so... May be worth holding out if you can.

I literally just got a Kobo Libra Colour a few days ago. Works great so far. I wouldn't recommend it for your back pocket though, too large for that. I think they offer smaller e-readers, but I'm not sure which ones might fit well in a pocket.

I quite liked the Pocketbook readers

To play devil's advocate, is there any way you might be able to fix the Kindle? I've saved two for family members by swapping batteries and it's not that hard if you have a spudger and decent set of screwdrivers.
- It has been slow and doing weird stuff for years now. It's first or second edition so it is quite old. I don't know if a battery change will fix getting stuck in boot screen.

Boox go 10.3, check mydeepguide on YT for a review.

I just use an old Kindle, but I'm considering buying an SPC Dickens Light 2 Pro. They're sold out, right now.

Lenovo m9 with readera app and a pirate library
- This but a 7" tablet. And man, if I could find a current one that ran an older version of android that didn't have a screen lock enforced, I'd buy 10 of them. I hate waking up in the middle of the night and having the full brightness of the lockscreen in my eyes until I can swipe and get my max-dimmed FBReader screen. Never had that problem on Android 4.X
  
  My m9 and s20 respect my brightness settings on the lock screen. Have you gone through the settings to see if something is forcing your lock screen to stay full brightness?

I just use the free kindle app on my phone or ipad
- I use the app too, but I don't want that as my main reading device because e-ink feels way more comfortable on the eyes.

i have a kobo libra 2 and it's served me very well. I sail the high seas for most of my books and I haven't had any issues at all loading content onto it, it's always fast and responsive, and I cannot understate how nice having physical page turn buttons is. highly recommend if you can find one, though theyve been mostly discontinued to make room for the color model that came out recently (would not recommend the color version, color e-ink is very hit or miss right now and it's not worth the screen quality trade-off).

You might possibly be interested in checking out the YouTube channel MyDeepGuide. Guy does in-depth reviews ofbmostly e-ink writing tablets, which perhaps is more functionality than you're looking for, but some of those manufacturers also do standalone "just" readers. Or maybe you'll be convinced to get a writing tablet :p