How do you handle backup?

I'm running my media server with a 36tb raid5 array with 3 disks, so I do have some resilience to drives failing. But currently can only afford to loose a single drive at a time, which got me thinking about backups. Normally I'd just do a backup to my NAS, but that quickly gets ridiculous for me with the size of my library, which is significantly larger than my NAS storage of only a few tb. And buying cloud storage is much too expensive for my liking with these amounts of storage.

Do you backup only the most valuable parts of your library?