The issue
I closed the issue. I hope others will find it and draw their conclusions from this little conversation we had. I'm not gonna argue with tests performed by someone who manages both the client and server sides of the system.
@tessus thank you for the help! 
Please consider adding a note to the FAQ describing this issue.
I see that there are things about slow sync and Nextcloud, but those seem to be outdated.
The next steps 
This leaves me with the following options:
a) re-introduce the dav2fs workaround (no chance)
b) use the Nextcloud sync client (tried that, had what seemed to be race conditions, I also suffered from data getting reverted [Sometimes latest changes get reverted (during sync?) - #9 by tangodelta])
c) just use this slow as molasses WebDAV solution
d) try the S3 backend
I don't have the resources to go with d), so I'm gonna go with c) and when things stabilize I'll try d). Seeing how Nextcloud caused issues for me in the past, I'll try to migrate away from it.