The majority of gamers game at 1080p. Both on PCs, and especially on consoles. Most people's TVs aren't even big enough for people with average eyesight to see a difference between 1080p and 2160p.
So the question to ask is if the steam deck is too slow, because the steam machine at 1080p will solidly beat the steam deck at 800p.
If you want something faster for desktop, just build a matx mid tower with a 9070xt. It'll cost double, but you'll be able to game in 2160p.