If I build you a custom device that receives questions from a central computer and lets the user plug in whatever keyboard they want via bluetooth or usb to only type and answer the questions (ability to edit the text and submit when ready). The central computer can receive all the questions submitted to any of these devices connected to it via wifi or cloud. How much would pay per device? Would you pay for a subscription?

I think this can already be implemented through a fairly mundane thin client approach, such as Dell/Wyse type thin clients, citrix stuff, or an ordinary x86-64 desktop PC setup with an absolute barebones OS that connects a graphical desktop session to something centralized over a LAN.

Or you can do a much cheaper tiny esp32 typewriter with a basic display and a usb for a keyboard. But who wants cheap for education.

Frankly, the problem is finding someone with the knowledge to administer it.

Initial cost is almost certainly not a factor; the components could be so old as to be free.

You cant type fast on them either

You mean on mechanical typewriters? It would be hard to learn to press keys slowly and avoid rollover.

Manual typewriters are no longer being manufactured. Several brands of electric and electronic typewriters are still are being made.