I previously synced Joplin with a personal instance of Nextcloud. I scrapped and rebuilt Nextcloud, starting over with a new instance and a clean directory. Trying to get Joplin to sync up again but it seems to think it's dealing with the previous instance, won't upload the full collection and gave the Fail-safe sync warning (so glad for this option).
How can I tell Joplin to just start over again and push everything up to NC?
The FAQ has a link to changing hosts. The gist of it is to sync, copy your entire host directory from one instance to the other, change sync target then sync again. FAQ | Joplin
I'm not sure if there is a force sync as such or not. I've seen other similar posts on the topic where the advice has typically been to export everything from Joplin as a JEX directory. Set up the new sync so everything is blanked then import the directory. You should obviously take a backup of everything first.
I agree, I came across the same situation once before but thankfully was only on a new instance I had set up rather than my main one. Basically make it so that Joplin forgets it ever had a sync target in the first place and syncs as if you just created one.
Ages ago I tried this as I beleive the local Joplin database has the notes flagged as synced so will not do so again.
On the client that still has the notes export all the notes as a JEX archive (File > Export all > JEX - Joplin export file). Then delete all the notes / notebooks from the client. Then import the JEX. The imported notes / notebooks should be treated as new notes and the client should sync them all back to Nextcloud.
Obviously not the best idea in the world to test on your "real" instance but I wonder what happens if one were to truncate the entire sync_items table from the db. This table is completely empty in a new/unsynced joplin instance so I wonder if removing these entries allows Joplin to pretend that the notes and resources have never been sync'd.
This would also help resolve synch issues if you are able to right click on folders and notes to push to server and then it pulls them down to other devices.