I always understood as a balancing act in your vdev sizing. Too big = long rebuild times with so many disks spinning up every time. too small = wasted $ with TB loss to redundancy (raidz2 with 3 20tb disks only utilizes 33% of the TB purchased).
I've always felt you should calculate how many disks do you need to saturate your connection and go from there. 10gig trunk on your network then you'll want 1 vdev to be able to saturate a 10gig line. any larger than that you don't get any benefit from.
synology 423+
does transcoding
synology automated backup of any/all computers on the network
synology drive works like dropbox
set and forget offsite backup with hyperbackup
can run docker containers and plex