Weird, I've been running and updating NextCloud since version 13, and I never had any problems as long as I didn't try to upgrade 2 release up at once and carefully applied the recommended database updates, etc.
I just spun up a new instance, I'll see if keeping it simple works for me. It's also on a VM now so I can take a snapshot and roll back to that if it breaks rather than relying on the faulty backup steps.