There's a few things at work here:
- Not much "hardware" RAID anymore because offloading works just fine and doesn't draw excessive resources.
- It sounds like you want to just take your existing disks and pop them into something else, which won't work.
- You shouldn't be running RAID over any external connections for a number of reasons if the coordinator (your machine) is hosting it. I can go deeper into that if you want.
You want a self-contained NAS that manages its own RAID and disks. I would honestly just get a diskless unit and start clean. You'll be better off in the long run.