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 😅

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.