If you use ELRS, probably 100ish km.

Yes it really depends on the transmitter protocol. I was using SBUS, simple, cheap, familiar from my previous project. But limited to < 1 mile or so I believe, but ELRS and other protocols allow tens of miles or more.

Isn't SBUS receiver<->autopilot electrical/data protocol instead of transmitter<->receiver radio protocol?

Yes, you're right, I meant ACCST not SBUS