For me, a top line QNAP can't compete with the mini & storage, & i certainly have zero experience of anything windows, but as always YMMV. If it gets sluggish (which I can't imagine) you just update the mini & if a hard drive dies, I just update the array. I'd always go down the road of using a mac mini as although the initial outlay is expensive for what it is, it's one less thing to go wrong - sure I could get a qnap or synology, but the chips in there aren't as powerful & although the mini has HDMI, I prefer the interface on TV. One look at the Plex forums would give the answer they're seeking & there's loads other posts on here talking about what people use - NAS vs mac (mini's are also mentioned as HTPCs) vs windows vs pi vs nvidia & so on. I'm not exactly sure what the OP's question is. You can still use the movies on the usb drive & plug direct into the pro.

So, I suggest unless you do have plex server on the mac, buy the pro. Plex still runs under Rosetta it's not M1 native yet, so whilst it's no doubt better at what it does than my 2012 (& loads of other competing options), I'm holding until either it's native or apple bring out the mini pro (hopefully both come at the same time for a full fresh install & rebuild of my libraries) I suggest you buy the nvidia shield pro, that can run plex server & client. Click to expand.I'd commented earlier that I was waiting for the new mac mini, that advice still stands, so that's my recommendation.

Shield pro plex server