Would the Mac Mini actually idle at that wattage if it’s open for connections? I doubt it, it’s probably more like 10W, which is generally the range for those smaller AMD MiniPCs or NUCs.

If it’s 10W, that’s a $20 savings from your NAS w/ a desktop CPU (and probably a discrete GPU, unless it’s running an APU). I can get 4% easily on savings, so I’d only need a $500 savings vs the Mac Mini to recoup that difference every year ($500 * 4% = $20). So if you already have an old PC, use that instead of buying a Mac Mini, and you also won’t have to fight macOS to do what you want.