I believe that converting your existing external drives to a concatenated set will require erasing them. As they are no backed up this sounds like it would not be what you want to do.
Also I would regard a concatened set as being at greater risk than several individual disks as a single failure would affect them all since the volume would be spread across multiple disks.
Your original idea of having a proper RAID protected NAS box is the best one. You can then configured iTunes running on your Mac to store everything on the NAS.
I'm aware that creating a concatenated set will erase the files. I would only do so after buying the extra 4TB disks and creating backup of all my files. I am also aware of the increased risk of drive failure, but this would not be a very high risk [(MTBF disk 1 + MTBF disk 2)/2] when dealing with a backed up set.
Still, at this point I'm thinking of maybe doing a concatenated 7TB set of two disks backed up to two separate 4TB disks via SuperDuper or CCC. That way the backups don't have the same risk of increased failure. Or maybe I'll just stick with 4 separate disks with the iTunes Library split and manually managed, as that will be the simplest, cheapest, and least complicated of all solutions.
The problem with a NAS using RAID is as follows: RAID = redundancy, but RAID ≠ backup, so I have no protection from enclosure failure, disk failure leading to enclosure failure (I've found recent horror stories on the net, mostly for Drobos), or software corrupting the data. The other problem is an empty NAS from Synology or Drobo costs the same as 3 external 4TB USB3 drives. And then I'd still have to buy the drives!
So I could have 24TB (12 for data and 12 for backup) in external HDDs or 12TB in a fancy case without backup but with redundancy. Or I could spend half the money, which is what I intend to do at this point. The question now is whether the concatenated set is worth it.
At this point, unless someone posts here with a horror story or links to horror stories when using USB3 for concatenated sets, I'll give it a try and update this in a several weeks.