Not sure about Unraid, but OMV will not create an array using drives attached by USB. If you really want to go this route, you need a DAS with it's own raid controller. I used this along with my NUC: https://www.terra-master.com/us/products/homesoho-das/d2-310.html
That being said, It was less than a year before I wanted to expand my storage and ended up building a box with room for 12 SATA HDD and a little intel 13400. If you have the money, I would skip the DAS and just build yourself a little server. If you don't have the cash or don't plan on filling those drives with a bunch of media files, just make sure your DAS has it's own raid controller.