Doug183,
At bit too many questions, and most beyond my knowledge. But, here it goes;
A 6 disk RAID-Z2 with 8 Tera-byte SMR disks should be good for data reliability.
Even if you store them on a shelf and just run a scrub once a year. My opinion
is that a 5 disk RAID-Z1 would not be a good idea when dealing with drives over
1 Tera-byte. Plus, I'd either store the disks in static bags and padded containers.
Or in enclosed drive sleds and padded containers.
For speed, if you start the copies as soon as the data is on your main pool, you
have the backup always in progress. The overhead of the backup should be very
minimal. The write speed of 4 data disks, (6 disk RAID-Z2), should be better than
my single disk 30 Mega-byte per second write speed. Don't know if you would get
120 Mega-bytes per second, (I'm a bit sleepy now, so my math skills are impaired :).
As for 2 copies, their is nothing stopping you from using LTO-6 tapes for one
copy, and a 6 disk RAID-Z2 as another copy. Always something to be said for
media diversity.
Last, are the Shingled drives suitable for your use? Most people here simply do
not have enough experience with them. And certainly not near your intended
use. Mine is cold storage / backups, so it's really only spinning during backups
and scrubs. Other than that, I just don't have the experience.