syncthing falls down when you hit millions of files.

Platform agnostic? Rsync from the perspective of duplicating the client-presented data.

Or rclone, another great tool. I tend to use this for storage providers instead of between self hosted systems (or data center fully self-managed systems.)

If the NAS uses zfs then zfs send/recv is the best, because you can send only the changed blocks. Want to redo the names on every single movie? No problem! I do recommend sanoid or syncoid, I don’t remember which. ZFS snapshots are not intuitive, make sure to do LOTS of testing with data you don’t care about, and which is small.

In terms of truly duplicating the entire NAS, we can’t help without knowing which NAS you’re using. Or since this is selfhosted, which software you used to build a NAS.