If you want it to run smoothly with video streaming and sane thermals/fan noise, I'd leave it with macOS. For NAS I'd instead buy a Raspberry Pi 4 or 5 and set it up with an external USB HD and Raspberry Pi OS.
Yeah, looking at the comments this seems like a better way forward. Re the NAS, I bought a bunch of devices in the past but their security was terrible, so I switched to an external USB drive which should make the setup you mentioned trivial. Thanks.