Hi Antares,
You can definitely run HexOS on the MS-01. Personally, I don't think the MS-01 is the ideal NAS device. You can indeed add extra SSDs via PCIe to M.2, but cooling will become a challenge. The MS-01 is widely used as a server in many homelabs. I have one myself—it's truly a brilliant device!
Minisforum did develop a prototype expansion card with four SSDs, but it was never released due to heat issues.
The great thing about the MS-01 is that it packs a lot of performance into a small case—actually too much for a NAS. At the same time, it has just too few expansion options for storage to be an ideal NAS solution.
But don't get me wrong—you can definitely run HexOS on an MS-01. With the setup you’re proposing, you’ll be good to go!