OK, the two most important links here:
-
Downgrade to version 5.600:
How to downgrade to an earlier version?
Android will complain that the app was built for an earlier Android version, just click OK on that. -
The best alternatives to K9:
https://www.g2.com/products/k-9-mail/competitors/alternatives
Somebody also suggested TypeApp, which is apparently a spin-off from K9 with similar looks, but the Play Store view only shows the unified inbox, I don’t know if they offer the account overview.
I just downgraded to 5.600, which works. Cost me half an hour.