Speed, Brake Pressure and Yaw would be the most important things I think.

I like the idea that the default position of the system should be "max downforce", and that it should only ever be in a non-default position when the accelerator pedal was at 100% for sufficient time recently.

A somewhat tricky thing is how to ignore gear changes.

Hmm, by introducing "lag" prior adjusting wing slightly longer then it takes for upshift, so that you are w/o aero adjustment on full throttle again?
Then again, in some tracks there might be fast speed shallow turns where one can stay on full throttle throughout with stock power level and good tires/aero. Imho switching off from full throttle despite flooring it might be wrong in this case. Then maybe custom aero adjustment tuning from GPS coordinates as input? Heck .. digging into too deep. And wouldn't have confidence to rely on it while driving 10/10.
I get overall impression, that active aero that gets auto adjusted is too overcomplified and not safe enough. And even if implemented, better with more/less button control from cabin, not from sensors/canbus reading to get pedal positions and so on .. but as on-the-fly manual choice of aero specific corner preoptimized, why not. Won't differ much from brake bias/engine map/TC/ABS choice for GT3 cars adjustment while driving, depending on corner configuration, how far into race thus weight/fuel, and condition of track/temps/rain/tire wear/pace.
But brakes position .. still useful, but better left for logging/telemetry purpose, to analyze driving and find out errors, what/where to fix with driving inputs, not as input source for active aero.

Deactivate wing movement when clutch is depressed. Working by GPS coordinates would take a lot of individual track tuning.

lot of .. but potentially netting best results possible. If something like dampers can be adjusted by gps (tein's EDFC), why not wing?