With vtx_max_power set to 2500 I had only 4 selections. 0mw, 25mw, 500mw and 1000mw. I was missing the last 2.5 watt selection.
Ardupilot smartaudio divides the 1000 range of my rc10 pot wheel by 6, this should give 6 selections but it gave me only 4. After many weeks of research (no one seemed to have my problem) I began to experiment. I thought that if I set vtx_max_power beyond the vtx max that it might divide the range and give me one or two more selections for my rush solo vtx 2.5 watt, or at least default lower to the rush solo's max (temp limit) wattage.
This did work and now I have all 5 selections!
My vtx_max_power is set to 2550.
vtx_options is 8, unlock only. It seems to make no difference but I leave it anyway.