Huh, I'd heard the name before but I hadn't realized how capable it was. Unfortunately when it comes to a data hoarder such as myself:

https://git-annex.branchable.com/scalability/

> Scaling to hundreds of thousands of files is not a problem, scaling beyond that and git will start to get slow.

So that's probably insufficient for me by at least a couple orders of magnitude. I'm able to maintain my sanity because snapshots simply capture device state, the NAS collects all snapshots while maintaining their independence, and (so far) borg has been sufficiently scalable to deduplicate any collection I've thrown at it.