That's really cool. I don't think you should support another keyboard, there may be better ones from a certain POV but I don't think there's a better one that's not significantly thicker and any keyboard options will quickly multiply complexity with any SBC/computing options. You will go down a rabbit hole if you start taking requests here. I think steamdeck internals plus rpi5 plus latest framework internals with no other configs is ambitious but not too ambitious.

My hobby is miniaturizing everything. If I were to fork this project (and I would love to after about 10 other projects are complete), I would think about selecting one good dongle, "shucking" it any other needed things, integrating them, and then finding the minimum volume to fit my preferred travel controller[1] and preferred travel mouse[2]. Then, I would consider customizing the housings of those things to be even slimmer without customizing any electronics except for maybe making sure everything gets charged while stowed. I would also consider minor mods to the keyboard to get rid of the bulk of the usbc cable. Pogo pins plus some 3D printing should do the trick.

[1]https://www.youtube.com/watch?v=55DO1HDeCHQ. No longer available new but this is the only good slim dual analog controller I have found.

[2]Still looking for a good one