Hello,
To implement the workaround suggested by kara I used this tutorial:
https://blog.sleeplessbeastie.eu/2017/09/04/how-to-mount-webdav-share/
and it worked OK for me. Remember to reboot after applying changes to your user groups (if applicable). Permissions AFAIK require a reboot to be applied. The drive is not mounted automatically, so remember to mount it or adjust the /etc/fstab entry accordingly.
Performance seems OK. Also: I tested what happens when you’re offline and try to sync: Joplin handles this gracefully.
As for stability and data consistency I have some observations but this isn’t the place to share them. I’ll post this in my thread about issues with data consistency.
Good job @kara !