PSA: If you haven't updated #K9Mail to 12.0, please don't update it. It will crash on start if you use the unified inbox. The bug has been fixed in 12.1 and will be available in the next #FDroid update. #Thunderbird is not affected, kind reminder to consider switching as K-9 will go away in the future anyway.

Workaround for now: Make a widget for a single mailbox, then you can open K-9, you can change accounts and disable unified inbox.