I own an HP Proliant micro server gen 8. Sweet machine. I bought it new, but there are plenty used around, great looking device. A real pro server in a small format. Celeron, but you can find i3, i5, or i7, and can take some specific xeons. 4 real HP drive caddies.an extra sata originally for a CD, that can repurposed for an SSD, and USB3 in case you want to add even more drives. Very low power consumption. I have xpenology installed, because of the polished experience, but you can install any NAS OS you want. Xpenology has a ton of apps you can install, and also does docker.

I would either get a used HP Proliant microserver Gen 8 (I own one. Pretty sweet) or get a mini ITX nas case, 4 or more bays. Depending on USB doesn’t seem like a good idea to me. You can always sell your mini PC. I have slowly learned to avoid the sunk cost fallacy.

The reliability could be hit and miss no? These could have a bajillion hours on them, depending on where they came from. 20 may be a bargain, until you have to keep replacing them every 2 years or less. Just guessing here.