I've seen a number of pictures of dual-screen clamshell type phones where the second screen is used as the keyboard, and I would assume a foldable could be used the same way? That's what got me thinking of checking into the models of phones available now with dual screens that might fit the bill.

I have a Sony Xperia compact (4.6" screen) that I designed and 3D-printed a custom case for which includes a slide out tray to hold a keyboard. I designed a keyboard circuit board with the assumption of connecting it directly to the USB port, but can't seem to find a suitable way to break out the existing USB-C port to provide a second very simple USB connection (without compromising the USB-C functionality of the primary port) that I could connect the keyboard to. My alternate plan is to upgrade from an Arduino chip to an ESP32 Pico D4 chip which could provide a bluetooth connection to the phone. I'm currently trying to wrap up some custom circuit boards for some computer control on a model train layout, but maybe I'll get back to this project once that one is finished.