Thunderbird gives me the irrites because of it's mbox storage format isn't robust. That's because isn't journaled, and POSIX append to file isn't reliable. It doubly irritating because their is robust alternative: maildir. Also it's google calendar plugin doesn't work if someone shares an invite with you. But on the positive side it's IMAP so losing local data isn't a disaster, and everything else about it is better than the proprietary alternatives.
BlueMail's nit is it isn't open source. Everything else is literally perfect. It's more capable the GMail or Outlook for example, which amazes me. I don't understand how / why they make it available for free, in fact it's downright suspicious, given it's so good I'd pay for it if there was a way. Still, I'd abandon it for an open source alternative if they did basic stuff like support subfolders. FairEmail doesn't. k9 was worse when I tried it.
I de-Google'd my life and Fastmail is amazing. I use their own clients because I'm creating new mail filters to sort my incoming email just often enough that I miss not being able to do that when using other clients.
I'm sure that's a pretty standard workflow.
At work, it's just mail.google.com.
Written in bash, python, and AWK.
About 2% to 5% of the time I use a webmail client, usually Roundcube.