Steam

That's a Steam Machine. The idea didn't take off because it's just a pre-built PC (and because they trusted other companies to do it for them). It's a highly competitive market with very low margins.

When they announced the Deck they did recommend one particular small box as a development machine as it matched the Deck's performance very closely. You might try looking for that. Although by now there are probably more powerful ones available for the same price.

I would commit some unspeakable acts if it meant Valve released a revised Steam Machine

You have it - it’s the Steam Deck.

Or release a SteamOS image so I can install it on my gaming rig.

I agree. It would also be nice to use as a pc ala the mac mini.

I run steam deck with a dock but it's closer to a switch than ps5 I guess. For what I play on a 1080 tv, with Elden ring being my heaviest title, it works perfectly.

I can see how a steam box that would deliver 4k on high end games would be awesome as well.