The tech is around, its just expensive. Im in the process of building exactly this right now with a radxa zero (4gb ram), waveshare 4inch lcd, 4000 mah battery using pijuice ups, and an aluminum shell from shapeways. There are rails on either side to attach joycons, and you can just attach one and rotate the screen for gba/gbc etc. It also runs moonlight client for playing pc games pretty well.

Its barely thicker than the switch, maybe 3mm or so. Once I get a better battery and wire the hat myself instead of using the gpio, it should be equal.

Batocera os with some extra shenanigans for responsiveness on screen rotates, etc

All in all the project cost between 200-300 bucks, which I think is fair.(does not include the aluminum shell since Im still using plastic until I get the size down) It might struggle for the highest end ps2 games, but I got it to run armored core nexus and makai kingdom so Im happy.

Theres also that clamshell, retroid pocket flip. It runs android and can handle a lot of ps2 games pretty decently.