Is Sony a6400 + sigma 18-50 f2.8 good for casual photography (as a beginner)

For fairly stationary things, especially if you don't have to zoom, a cellphone will get you most of the way there - unless you're going to be in a very dark environment. I say this as someone whose carried around a dedicated camera for a while, but the best camera in the world is the one you currently have with you. You can absolutely get in the habit of carrying a dedicated camera with you though.

80-90% SOOC (can't give much time to editing). I am ready to give time to learning and practicing manual controls though

Most camera brands and bodies will give you solid SOOTC JPEGs, but each offers some level of twist. This is where people start taking "color science". All the RAWs are basically the same, but the algorithms to make pleasing looking JPEGs vary by brand. Nearly all are customizable, so you can tweak one brands twists to be more or less pronounced. Based on my personal experiences I would say:

Fuji (X-H2s) = a bit stylized, but pleasant to look at. Reliable white balance and subject meeting, after changing the default metering mode.

Nikon (D40, D5300, Z6II) = warm and pleasing. The Z6II I owned for a little while would struggle with white balance indoors, especially with warmer interior lights. It also tended to meter the frame, not the subject, but you can customize this some. I have more than a few photos of someone underexposed in front of a sunny window

Sony (A7 III) = probably the most true to life, but true to life can be kind of boring/flat. The most reliable auto white balance and meters for the subject out of the box.

That's not to say that other cameras don't do a good job, I just don't have personal experience with them

Ergonomic, light.

Here I would say:

1. Micro four thirds. Smaller sensor = smaller glass. One of this system's selling points is it's size/weight, so they tend to not pull a Fuji
2. Fuji mirrorless. All they make are crop sensor bodies, so their glass is usually optimized for that sensor size. Meanwhile over im Sony/Nikon/Canon world, they make a mix of FF and crop sensor glass. Sometimes there's a FF lens that stands out and you wind up using it on a crop sensor body, resulting in a bigger-than-necessary lens. My D5300 nearly always has a FF 70-300 on it because the lens is really very good and when it came out there wasn't a crop body equivalent
3. Sony and Nikon mirrorless crop sensor bodies. You could also throw Canon in this mix. See above for reasoning
4. Mirrorless FF. See the A7III with the somewhat pancake lens on it in the link brlow
5. DSLR. Generally speaking the largest glass for a given focal length and aperture, but as people move to mirrorless it is getting cheap to buy this glass used...

Here's a rough comparison between all five using the focal length you referenced. I also snuck in Sony's 50 FE 1.8 on a FF camera to show that each system will tend to have a compact prime or three.

travel photos, capturing scenes like I see them with my eye

Most cameras should do just fine here IMO. Procedural photography has made strides in smartphones, but it's hard to beat a dedicated camera

Low light photos

Fast glass will make this way better. If you're serious about low light, stop thinking about a f2.8 lens and start thinking about really fast primes. If you haven't read about f-stops yet, the quick primer is f/4 to f/2.8 is one stop, f/2.8 to f/2 is another, f/2 to f/1.4 is yet another.

IMO you should buy a f/2.8 lens for any gain in image quality it offers over its f/4 counterpart - not because it's faster/lets in more light. If you want to really let in more light, a prime lens is the way to go.

Note that fast glass = shallower depth of field if shot wide open. This is potentially one of the advantages of something like micro four thirds. I took pictures of my kids with Santa this winter and stepped down to f/5.6 to try to get all their faces fairly sharp on my 50mm and A7III (full frame). I had about 0.4 meters of "in focus" plane. On a micro four thirds body, I could have used a 25mm lens, at f/2, and wound up with a very similar looking photo. Although the FF sensor is about a stop lower noise, the micro four third photo would have probably had lower noise due to the two stop faster aperture. Not that this particular photo is noisy, but you hopefully get the point.

Long-lasting gear. Okay with getting prime and telephoto lens later if needed.

Most gear will last quite a while, especially if you keep it dry. If it's going to get wet, make sure to get weather sealed. That's going to drive up cost.

Videography is not a priority as of now, may explore later.

Same situation here, lol.

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So, think about what you want and go from there.