I was going to mention ZFS, but I suspect Raspberries are too weak for ZFS?

If you can use ZFS in both sides, send/receive is the bomb. (I use it for my backups.) However, I’m not sure how well encryption would work for your purpose. IIRC, last time I looked at it, if you wanted an encrypted replica, the source dataset should be encrypted, which did not make me happy.

I’d love to work on making NASes “great” for non-technical people. I feel it’s key. Sending encrypted backups through peers is one of my personal obsessions. It should be possible for people to buy two NAS, then set up encrypted backups over the Internet with a simple procedure. I wish TrueNAS Scale enabled that- right now it’s the closest thing that exists, I think.