I’m considering sync for version control and redundant back up (I only use one device). My vault is on NextCloud. Will that work?
No. Sync is for syncing between devices.
The snapshots are meant for rolling back if you make a mistake. They are not a full robust version control like git or SVN or similar.
And syncing tools in general are not sufficient as full safe backups. If something goes wrong with your data, syncing tools will sync those changes to your other devices. (For one example, sometimes when power goes out, files that are being edited at the same time get corrupted.)
As for working with Sync + NextCloud, I’m not familiar with NextCloud. But mixing tools that read+write files is a recipe for data loss and file conflicts. If you were making a one-way copy of your vault into a 2nd cloud tool, then it would likely be fine.
So no, it really sounds like the way you’re imagining using Sync isn’t feasible.
Thank you for the clarity.