Was actually confronted with the same problem - thank you for the solution!

Aegis authenticator. Beats all proprietary apps I’ve tried so far

I’ve been contributing to it via Street complete (can get it on fdroid) and it has become quite a hobby of mine.

Maybe Citizen Sleeper, but its very text base and plays more like a ttrpg

Depends ok the cloud solution you use, but it definitely could be an issue. Probably not because of locking, but because the files probably aren’t locked with the clientson there could be issues while merging the changes.

It seems to be file based, so wouldn’t it be able to use other self-hosted sync-tools like the next cloud client (given you’re hosting your own instance)?