Same for me. I'm a die-hard Thunderbird fan (it's ugly but it works lol).
Used to use TB at work until we switched to Google Workspace and they globally disabled IMAP access. Now I'm stuck with webmail and my productivity went to absolute shit.
I haven't but I definitely should. Just refreshed my laptop with pop os and have been using the default mail client with it (Geary?). It is really responsive and works well with the tiling plugin.
Yeah, I mostly use Thunderbird and FairEmail but it's nice to have a backup when I'm in places I can't use those, or can't be bothered setting up a new client.