I’m also on K9, while not the prettiest, it’s much better than it was.

If you weren’t aware Thunderbird and K9 have teamed up. On the roadmap for this year including updating the message view and a major overhaul of the UI:

The K-9 interface looks outdated, does not conform to the current material design (Android platform) standards. We intend to bring it up-to-date with these as well as fix some workflow issues. https://developer.thunderbird.net/planning/android-roadmap