I ran into the same problem some months ago when my cloud backups stopped being financially viable and I decided to recycle my old drives. For offline backups mergerfs will not work as far as I understand. Creating tar archives of 130TB+ also doesnt sound like a good option. Some of the tape backup solutions looked to be possible options, but are often complex and use special archive formats...

I ended up writing my own solution in python using json state files. It's complete enough to run the backup, but otherwise very work-in-progress with no restore at all. So I do not want to publish it.

If you find a suitable solution I am also very interested 😅

If this fits your budget (you still need the actuals disks..) it's not a bad choice. Speed should be sufficient for HDDs, as it's USB 3.

As the other poster suggested, don't use its hardware raid. Use it as a JBOD and configure the raid in Linux with ZFS or similar.

And never forget: RAID is not a backup! You still need to do regular backups, at least for important data.

Honestly, unless you can spend more $, one or two USB disks for the mini pc is probably your only choice.

I'm surprised no one mentioned ansible yet. It's meant for this (and more).

By ssh keys I assume you're talking about authorized_keys, not private keys. I agree with other posters that private keys should not be synced, just generate new ones and add them to the relevant servers authorized_keys with ansible.