[Someone]

SMS is stateless, so you’d have to add sessions on top of it. The simplest probably would be to have a single endless session tied to your IMEI or phone number.

For UI, I would look half a century back. ed (maybe with slightly more verbose prompts. Sending a single character is as ‘fast’ and ’cheap’ as sending 100) may well be the editor of choice, for example.

Alternatively, look at speech interface systems, and adjust them for text input. Those typically are more constrained, though.