I'm still not done with the testing. I actually had a setback: I borked the data and had to restore from backup (always back up, kids!). So the results won't be coming in soon.
But one question popped into my head in the meantime: is it normal, that after switching sync methods Joplin wants to sync everything from the ground up?
I went from file-system sync pointing to a folder managed by Nextcloud Desktop Sync to the Nextcloud back-end in Joplin and Joplin wanted to sync all. I allowed it but it was extremely slow (about 70 notes in 15 minutes) so I aborted.
Shouldn't Joplin just "get in touch" with the backend and sync only what's missing? If there is anything to sync at all?