I’m not sure what a Steam deck costs? $450 - $550?
A Minisforum UM870 with 32GB of memory and 1TB SSD is around $500. That has an AMD Ryzen 7 8745H CPU. 8 cores 16 thread 4.9ghz boost Zen4. Gpu builtin Radeon 780M with 12 RDNA3 2.7ghz cores. That could give the Steam Deck a round for it’s money I think.
It’s really amazing how much power you can get for cheap in a small package these days. But yeah putting a battery on that wouldn’t be practical, it’s still a very efficient chip, but in absolute terms eats a bunch of power. So you’d need to be tethered. You could easily wear one of those mini pcs, but need a power cable. One of those could be dropped down from the ceiling, but still not as good as a mobile solution.