IIT: People who've never used Gmail complaining about how it's full of ads. They only appear in the "promotions" tab, which is where Gmail sends stuff that's already borderline spam. In other words you don't see ads unless you go looking for them.
I don't see anyone else here recommending K-9 Mail for Android, and I think that's sad because it's really good. Open source, supports viewing message headers, multi-account, hides remote images by default to prevent tracking...
Google knows what you like and these days they will take any ad because they fired all the screening staff. My ads are usually pretty average since I opt out of everything I possibly can opt out of and I use Startpage for search, so they aren't as targeted. That's one positive thing about Google. They started as a relatively ethical company for an ad company, so there's a lot of code and best practices in place for opting out of things. That is fading, but it's way better than others. Like Facebook showing ads for things I searched for a few seconds ago on Amazon and stuff like that.
Spark Mail (ignore the "AI Email" thing in the store listing, it's not intrusive or even a major part of the features) is a nice app and works with multiple providers. It even syncs which accounts you have added across devices.