I've been using FairEmail[1] for some years now as a replacement and find it superior to the gmail app. Of course, depending on your needs and tastes, I could also understand calling it a bit clunky. It is FOSS, but has a one time pay premium option for some advanced features. But really, it's also just fair to support the dev by buing the app. My only complaint would be, that there are to many updates, but of course, you can just ignore them and do them every few months instead.

[1] https://email.faircode.eu/

thunderbird (formerly k9 mail) is a decent enough android app, but im not very picky when it comes to email either so keep that in mind. ive been using it with posteo for about 2 years now