@j I've used K9 in the past, and it's currently installed on my 'phone with accounts set up (with manual sync only), but it's not my main driver.
My main e-mail app is FairEmail, I have the Pro version and I've used that for a few years and it has an excellent feature set. The only concern (for me anyway) is that the creator of the app (Marcel) is very sensitive to criticism, even if that criticism is not warranted. He gets very upset to see his app being criticised on the Play Store, and frequently has intimated that's he might close the app down.
Should that ever happen, I will revert to K9 - it's been undergoing some updates recently and I believe it may be renamed to 'Thunderbird'.