I still very much think that non-RAID storage, with a similar duplicate amount of capacity for full backups, is the best option.

RAID is not a backup, so if one goes with RAID1, then that doubles the drives cost (primary plus backup arrays), versus the non-RAID scenario. And non-RAID with backup protects against more failure modes (software, human error, ransomware..) at the slight expense of not being up-to-the-minute on the backup.

EDIT: unRAID of course tries to split the difference here, giving most of the properties of a RAID scenario without having to duplicate every drive. Kinda like RAID5, except the individual drives are still accessible in the event of an array failure.