GPD already makes such things, as does ClockworkPi. The Flipper One is exciting in significant part because it offers a different, smaller form factor.
Wish there were better options than those... Other than the uConsole, handheld computers with thumb keyboards seem to be in the category of literally handmade cyberdecks.
https://hackaday.io/project/202522-cyberdeck-handheld
https://youtu.be/u8kYHgKKhjY
https://youtu.be/IB5OJr_MimQ
Trying to make my own too. Ordered some prototype keyboard PCBs. No idea if I'll get anywhere but I'll try.