If you're interested in open handheld devices, the Librem 5 smartphone has its layouts, schematics, assembly drawings and 3D designs published on a free license:
A much simpler alternative is a $25 R36S handheld console (more powerful 4 core cpu, runs both debian and android) and print a different case with a keyboard.
Link to a suitable keyboard? Seems like an interesting idea.
[Edit] Oh, I see. You can get them in retro Gameboy shapes, so then any of the BlackBerry keyboards would work.
I'm old enough to have purchased an Open Pandora and seriously considered """"pre-ordering"""" a Dragonbox Pyra. I'm so glad I decided to hold off until launch because well would you look at the time! It's been twelve years.
And the pandora took years to arrive, got constantly more expensive while waiting, while the hardware got pretty long in the tooth. Still, it was a greadt device! But when it arrived, it was already a curiosity from another time and I feel this even more so for the Pyra.