Like others. I back up the NAS to a second NAS. I periodically (a couple times a year) backup the irreplaceable stuff to a hard drive in a fireproof.
I use the NAS mostly for convenient archival of a lot of digital junk. These days, precious photos have multiple cloud backups. Important work stuff is backed up to two different cloud services as well. I Plex'ed my media and donated the physical copies to charity. I'd be pissed if I lost it, but it's replaceable with cash, so I sleep ok.
I have found that the more NAS I have, the more junk I keep, so I stopped adding storage at 18TB. There are 18TB external HDDs these days, so I'll pick up a couple of those and start doing rotating offline backups.
Sometimes this all feels like a part-time job without pay...🤪