Usually the radio is fixed-band, but the receive and transmit digital side will usually be some combination of fixed-function pipeline and actual DSP software. The problem is that the information needed to reprogram them is highly proprietary.
Yep, you need to be DJI or similar to get access to them. Very discouraging as a small fish. (I've looked into the broadcom-based project above, unfortunately it's not suitable for continuous reception/transmission, and you would still need processing power anyway).