Along with seemingly everyone else, I had a quick look at my options to migrate away from Apple over the last few days. First thing I looked for was a decent mail app, was immediately put off the whole idea, exactly as you describe.
This is probably the most painful thing on desktop at the moment, because no one gives a s* to desktop mail apps these days.
I recently switched back to evolution from Thunderbird, but where one if great, the other falls short ( in this case, GPG with Evolution is _painful_ ). Geary is... well, not mature, Claws and Sylpheed are a bit too oldschool and mutt is ... well, mutt.
I'd love to see something like Rainloop[1] as a real desktop app. Mailpile[2] is a nice idea, but is nowhere even close to the usability of Rainloop.
[1] https://github.com/matzipan/envoyer/blob/master/README.md#re...
As for mail, Thunderbird is not so bad.
I'm still on the very same install, just switched to xfwm4 and tint2, and I'm quite happy.