Just install Linux on the old rig.

Does it have a proximity sensor on the power screen? If it is only a second screen, I can see an easy curtain hack.

My guess is they’ll enforce a xiaomi-style thing where you have to login, wait, go through hoops, wait again,…

my ubuntu laptop is running Microsoft Defender

How does that even work? Just wine it?

A desktop running a low usage wouldn’t consume much more than a NAS, as long as you drop the video card (which wouldn’t be running anyways).

Take only that extra and you probably have a few years usage before additional electricty costs overrun NAS cost. Where I live that’s around 5 years for an estimated extra 10W.

Clean the PC and change the fans. Fans make noise when old.

It’s still good PR for linux

Yes, with dosbox.