I think we all struggle with storage
Currently using 4 X 4TB external WD Drives via a USB3 HUB and a couple of TB locally on a PC (Cataloged in Coollector). Looking at a secondary backup on these and expansion routes isnt clear at the moment though. If i went Large NAS (as you have gone) then i would be looking at mirroring or something like RAID 10 and there is a large overhead on capacity as a consequence. I suppose with any storage requirement there is a big $$ cost associated with multiple copies/disk redundancy/hot swaps that some will willingly pay and some won't.
Have a good one