Thanks a lot for the super detailed reply, it’s genuinely helpful and clears up a lot 
I do have a few follow-up questions though:
1) Difference vs ZimaOS built-in Backup (Auto mode)
How does your approach (rsync + cron + container) differ from the Backup feature already built into ZimaOS, especially the Auto mode?
In the UI it says: “When the source is Zima Storage or a USB drive, backups run instantly whenever a file changes.”
So I’m wondering: doesn’t this already behave like “sync on every change” (or close to it)? If not, what’s missing / what makes rsync the better option for my use case?
2) Rsync with a GUI
Is there any rsync solution with a GUI that can be installed (ideally from the ZimaOS App Store)? I didn’t find anything there, and I’d prefer not to manage everything purely via command line if possible.
3) HDD rotation while using the same bay
Ideally I want to rotate my backup HDDs by swapping them in the same bay, so I never have an empty bay and I don’t have to keep reconfiguring paths each time.
Does ZimaOS (or rsync with your setup) automatically understand that the drive inserted in that bay is a different disk, and then restart an incremental backup correctly?
For example: I reinsert a drive that hasn’t been synced for 3-4 months, so its backup state is older and “out of date” compared to the SSD RAID0. I want this to be fully automatic: swap the drive, and it resumes by reconciling differences (not starting from scratch).
Thanks again!