As you say Thunderbird seems stalled, but despite that it continues to work. So are there specific annoyances you have with it? Or think you'd like added/changed?
Most of the big (graphical) mail-clients are very similar in terms of featuresets, if you ignore the GUI toolokit and similar distractions.
I personally prefer console mail-reading, for many years I used mutt, but then I started to get annoyed that I couldn't script it terribly well. So I wrote my own mail-client, console-based, modal, and scripted with Lua. It works for me, and even has IMAP support, but I suspect it is not so useful for you:
You can’t ever delete mail from gmail. Only hide it, at best.
Gmail only ever has one folder, into which everything gets thrown.
https://www.mozilla.org/en-US/thunderbird/52.5.0/releasenote...
My second option, beacuse of the features it offers, would be Kmail/Kontact.
Claws-mail is pretty good too, if you need something light yet functional.