Cheers - I cant 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.)
Not necessarily as you expect the drives to fail especially with a RAIDZ2 or 3 which is why we use RAIDZ2 or 3 or have hot or warm spares. You have some fault tolerance in that configuration. You may also have to in a pinch build a NAS out of desktop hard drives or use a desktop drive for disaster recovery as a temporary drive. However, enterprise or NAS drives are designed for a NAS and should be used for that purpose but IMO it's not a NAS breaker or disaster.So I have seen in a few people's build signatures, they are listing Desktop drives as in use in some of their array's. Isn't this bad juju with ZFS / Freenas?