To allow failover between heads in active/active pair each head must be up and running; and to be able to run, each head needs at least root volume. Your configuration leaves second head as cold spare. It could be acceptable if you can afford downtime for replacement (I had customers with single FAS250 or R200), but it is pretty expensive spares stock IMHO.
As noted already, you need a root volume for each controller. So best bang for your buck would be a 3 drive Raid-DP on the 2nd controller ( 1+2 no HS) and the other controller would have a 7+2 (raid DP) No HS, or a 6+2 DP and 1 HS, your call
Various replacement procedures are described on NOW site.
I really can’t say which is the best. In similar situation several years ago customer opted for hot standby with 2 disks in RAID4 for root and no spares on one controller, but they were 72GB disks; I guess you would not like to lose 4TB worth of space. If you do not absolutely need large contiguous chunk your suggestion looks reasonable; although I’d rather go for RAID_DP without spares.
Search communities, this question pops up pretty often and there were several long threads about it already.