Ready for expansion (RN Pro Pioneer Edition)-Seeking Advice

I was assuming you had a four slot unit. You can get more space by using your two empty slots. If you add 2 more disks ( 1 TB or greater) then your array will expand by 2 TB, and any extra will be wasted for now, until you upgrade the second pair to the larger size. You are still subject to the 8 TiB growth limit, so you couldn't expand the volume beyond 10 TiB unless you do a factory reset.

So then the question becomes how big you want the larger drive size to be. It is more cost-effective to use bigger drives if you look at GB/dollar. But of course the drives do cost more. Also, your storage needs appear to be pretty small, since the 2 TB volume you have now has lasted you 4 years.

So options are likely 2 TB drives or 3 TB drives. At current pricing if you use WDC Red drives:

(1) adding 2x2TB will cost $220 (Amazon). You gain 2 TB of space - double what you have now.
(2) Upgrading 1 pair costs another $220. You can another 2 TB of space.
(3) upgrading the last pair costs another $220. You gain 2 TB of space.

So the total project is $660 for 6 TB.

Doing the same with 3 TB drives gives you
(1) Adding 2x3TB costs you $260. You gain 2 TB of space
(2) upgrading another pair costs you another $260. But you gain 4 TB of space.
(3) upgrading the final pair costs you another $260. You gain another 4 TB, but you need to do a factory reset.
(3a) upgrading just one of the final drives costs $130, and gains you 2 TB without the reset.

Stopping at step (2) gives you 6 TB of growth for only $520 - $140 cheaper than going with 2 TB.

Basically the price difference between 2TB and 3TB drives is small enough ($20 at the moment) that you might as well go with 3 TB.

If you want to stick with enterprise drives, you can do the same math easily enough with whatever models you want to use.