I don't remember why but I switched from k9 to fairemail a long time ago. I think it might have been the branding lol... Shallow I know but I just can't get passed the logo / name. I might give it another shot once it's rebranded.
I love FairEmail because of its "millions" of settings and the privacy features, for an example if you press a link, you'll get a popup with options (for an example, what app you want to open the link with). And if the link contains trackers, FairEmail will remove these by default and saying "tracking parameters removed" with yellow text in bold.
K-9 Mail feels incomplete in comparison. Have you tried FairEmail?
And when was "last time"? :) I have been using F-Droid for FairEmail since I tried the email client for the first time few years ago and I have never got any issues. Just updated FairEmail while writing this comment. Works just fine :)
K-9 mail is what I originally used, but it isn't supported or being developed any more. There were some weird issues that I can't remember now that caused me to switch to FairEmail.
Having said that, I almost switched to FairEmail because K-9 lacked support for some sort of authentication measure (which I no longer need), but that wasn't because K-9 stopped development.
I don't recall it ever being a dead project. They did have a time period where you had to either join the beta on Play Store, obtain the beta on Github releases, or use F-Droid and install the beta. They were working on integrating certain things and rewrites before doing an official release.
It was a pinned issue in their issue tracker.
The whole 5.7xx series were betas, and 5.800 started the official releases again