Honestly? I think you are getting hung up on approaching this from the T9 perspective.

Yeah well I think you are hung up on the mechanical keyboard community.

The T9 approach worked excellently, was extremely intuitive, and once learned, it is easily as effective as an on screen keyboard.

There just aren’t enough buttons on a deck to have dedicated key mapping and as well would never be intuitive.

This can be overcome by incorporating the philosophy of the T9 keyboard, taking it down to maybe 8-12 total keys you interact with.

Two hands, two flippers (index fingers), 4 keys either side (two thumbs), and you’ve got all the degrees of freedom you need.