[Moru]

Have you tried Resilio Sync?[0] I'm using it on Android but seems to exist for iOS [1] too. I'm not syncing the library though, might want to make sure you only run the sync when Calibre isn't running to prevent data loss.

[0] https://www.resilio.com/individuals/

[1] https://apps.apple.com/us/app/id1126282325