My guess is that this is 100% attributable to riding the brake for an extended period. If the pads are almost but not quite engaged (which is how they're supposed to be), then nothing is happening. There's nothing in that to warp the rotor. But if you ride the brake for a long time, and the rotor doesn't have a chance to cool off, that could warp it. I don't think loosening the calipers would have any effect, although it could be useful as a diagnostic measure to see if the rotor is pinging intermittently on the pad, or continuously. If it's continuous, you need to recenter the caliper (easily done). If it's intermittent, the rotor is warped, and needs to be trued or replaced. I don't think different pads will make a difference—you still need to dissipate the same amount of energy. A different kind rotor might, but I'm not aware of any high-heat rotors. You could also probably put the caliper on a different adapter plate (or flip the one you've got, not sure) and get a bigger rotor, which would allow that heat to radiate away over a larger surface area. Road and gravel bikes are often set up with 160-mm rotors front, 140 rear, but you should be able to put a 160-mm rotor in the rear.
Also, I would call a mile at 10% a long, steep descent. I alternate front/back every few seconds in situations like that.