This would be a great use case for ZFS. You can use dataset replication to sync the data. I’ve been very happy with TrueNAS but I think it only became recently available for the Raspberry Pi so not sure I would recommend it but I think you could use ZFS. You can use ZRaid too if you multiple drives. The only limitation you might encounter is memory since ZFS is a bit of a hog. On the plus side though, ZFS should protect against bitrot if you’re worried about that.