I don't disagree that the radio is used (freq changed) frequently over the course of a flight. However, the primary radio freq and standby would be easily shown on the top of the PFD.
Also, considering the ease of pushing the digits on the transponder, it quickly shows the poor UI of knobs for entering the freq on the radio. However, I just realized you don't have enough buttons at the bottom of the SV to input the freq, so I guess a separate head is needed.
Maybe a "FMS" style panel that could be used to input the freq, "qwerty" keyboard input txt for flight planning, etc.
As you design the head, can you consider a row of buttons to easily input the frequency versus the 100 year old UI of knobs?