Cheers - I can't afford the type of disk resiliance that you have but I'll look at getting another 8TB.I'd say it is up to your risk tolerance.
I have my FreeNAS using NAS drives (WD Reds) in a RAIDZ2 because I want the protection. That is why I back up using rsync to my old Synology NAS (mirrored drives) and then to another, external USB drive. NAS (or Enterprise like WD Gold, Seagate Exos, etc.) are made to be powered on all the time. (I'd stay with NAS drives and the performance of the Gold/Exos drives likely isn't needed in most cases.)