You’re far better off having two raids, one as a daily backup of progressive snapshots that only turns on occasionally to backup and is off the rest of the time.
I don’t understand how it is better to have an occasional (= significantly time-delayed) backup. You’ll lose all changes since the last backup. And you’re doubling the cost, compared to just one extra hard drive for RAID 6.
Really important stuff is already being backed up to a second location anyway.
Really important stuff is already being backed up to a second location anyway.