Kodi on a Raspberry Pi 4 is pretty good, and you can run moonlight from within Kodi. Of course, you have to find a reasonably priced Pi…

EDIT: Also, I use this setup in a spare room, but use an Apple TV in my living room for identical purposes. Though I use Infuse and the actual Steam remote play application.

i mostly use windows on the tv, an old laptop and a wireless kb/trackpad. runs cool, even with the lid closed all the time. reliably wakes up outputting to the external 'monitor' as long as it hasn't 'lost' its connection to the tv (can't unplug that input temporarily for another device--would have to flip open the laptop lid later to re-enable the external-only setting), and it's new enough (skylake) to have decoding in hardware for the formats i've been using.

the other hdmi is a linux sff desktop set up similarly, but i haven't got all the kinks worked out yet.

use firefox (with addons and some userscripts|styles) for online sites, and usb externals or lan connection for 'saved' content. shortcuts on the desktop (to web sites, playlists, lan shares) function as the 'home screen' or 'launch' page.

i do have the tv 'online' atm, though, because a lot of pluto 'live' tv get watched here, and the tv 'app' is faster and more reliable than their web site. the remote has a hotkey for it, too, which goes directly to the last 'channel' it was on.