but before I go ahead do I need to do any/all of the following: Defrag,
Balance
Scrub
Disk Test?
and if so, in what order?
Well, there is no definitive answer on this. If the disks and volume are healthy, there's no need to run any of these. I do suggest running the vendor diags (seatools for seagate, lifeguard for western digital) on the replacement disks if you can.
Personally I wouldn't do a Defrag or Balance first. I think they run faster if there's more free space to work with, so it's best to postpone them until after the volume expands.
If the existing disks are healthy you could do a scrub - that confirms that all the RAID parity blocks are correct (and those blocks are used during the resync after you replace a drive). But I think it's pretty rare for a scrub to find an issue there.
If you are concerned about disk health, you could do a disk test instead.
If you want to do both a scrub and a disk test, then do the disk test first.
FWIW, when I need to expand I generally factor in the cost/TB gained as well as the total cost of the upgrade.
Assuming WDC Reds (and current US Amazon prices), you'll pay about ~$260 for 2 TB more space ($130/TB).
If you went with larger disks, you'd pay more now, but the cost per TB gained would drop. For instance, 2x8TB would cost ~$480 for 6 TB more space ($80TB), and also simplify expansion later on. One limitation of the RN104 is its slow resync time, but resync of a 16 TB volume should be acceptable.
