I used to use K-9 regularly, years ago, but yes — it seemed to be unmaintained, and when Fastmail's own client got to the point of not sucking, I didn't have much use for K-9 any longer.
I used to be quite fond of Thunderbird as well; it's been my daily driver since 2004. I still use it from time to time, although not as often as I use Fastmail's web client. Thunderbird feels like abandonware, though. It gets updates all the time, but none of them seem to change anything noticeable, including long-standing issues like its tendency to get confused by search terms and its unwillingness to play nicely with high-DPI font scaling. And I'd love to see proper JMAP support added..