A second nas. Both my nas' have raid. They also have revisions, and one is off except for when running backups.
A lot of stuff has a third copy on the server I run stuff on.
I may some day take your approach and make the backup nas smaller, then exclude media I have in torrents. Though restoring 10 TB over i2p may take a literal year in case of failure.
PS: the backup raid is btrfs raid :)