Debian will work just fine if you want to use that. It's certainly stable enough.
Go for it if your complexity is two mirrored disks. Anything seriously important stick it into a directory to be backed up off-site with incremental backups or however you'd prefer to sort that out.
Yes it is.
Use to run Arch Linux with ext4. Now run FreeBSD with ZFS.
I’ve spun down drives in TrueNAS, there’s definitely options on a per drive basis
Would you happen to remember where that is in the settings? My research so far indicates it's not really a thing because of ZFS, like someone else also commented...
If you go to Storage -> Disks -> Edit there’s HDD Standby and Advanced Power Management, should spin down stuff
I'd already set that to econ/quiet settings, unfortunately I realized I still had stuff running off the main drives - I'd need a second SSD to create another filesystem on, and I'm not prepared to do that just now. Thank you still!