And yes, synology allows you to 'grow' existing arrays - so you could put the current 2 * 16tb in a 4-bay as a raid1 array, then later add another 16tb to go to raid5 ( taking it from 16tb usable to 32tb usable ) and then add a 4th to either go bigger again ( 48tb ) or increase redundancy by going from raid5 to raid6 ( although raid6 on a 4 drive array doesn't make much sense - might as well just run raid10 as it'll yield the same space and have better write performance )
this is one compelling reason to head in the synology direction. i think it was asked within the thread but I did not address: do I really need boat-loads of storage ? -- answer: no, absolutely not. at least not in the context you might see based on your experience.
i'm a small-time home user, with a family. and maybe 2 or 3 users at home at most are queuing the NAS for something, mostly media content. MOST times, it's sitting idle and NEVER will see anywhere near the 24/7 use of a data center. that's not to say that the NAS isn't putting the drives to work at all when no one is accessing data, but most times it's crickets in the NAS.
if i threw 2x 16TB + 2x 8TB + 2x 6TB into a larger NAS, and came away with a single 16TB volume, I'm happy with that, knowing I can expand later on and not have to fiddle with drive swaps or waiting days to copy all my data from one enclosure to another when upgrading. as it stands right now, the WD EX2 Ultra is so barebones, it does not offer any kind of upgrade subroutines for drive swaps. you simply backup your junk, put new drives in the NAS, and restore your junk back to it from the backup. if we are talking 16TB worth of junk over usb3.0 connection .... it amounts to the rysnc job taking about 4 days to complete.
imo the synology would be an upgrade by at least a few clicks