I use Thunderbird and hate it. Full of bugs and cramdowns (not sure what the right term is) where they make a bad setting that you can’t undo. Also too difficult to find too many things. And way too many non-mail features. I haven’t bothered seeking a good alternative but Thunderbird leaves a lot to be desired. Forking isn’t likely to help much. It needs total replacement.
I’ve been using Debian MATE. It’s mostly ok.