Mulberry (https://www.mulberrymail.com/) is, IMHO, the best IMAP client ever written, but from an IMAP perspective (and not necessarily a GUI perspective).
I gave up on Thunderbird when it started to demand most of my system memory. I switched back to mutt.
Drat, the best of the bunch was Eudora. But when Qualcomm abandoned it and turned it over to Mozilla, it became a lost child, stranded between its former self and Thunderbird.
That Eudora and now Thunderbird should be end-of-lifed makes for very few good free options.
Even Ubuntu, which replaced Pidgin with the very inferior Empathy so they could be more "Gnomey", gave up on Evolution. I don't know that there's too much hope left for that one.