I also wouldn’t be surprised if it was possible to create a specially insulated shirt that has a personal chiller (perhaps backpack size) piping cool air down y

I also wouldn’t be surprised if it was possible to create a specially insulated shirt that has a personal chiller (perhaps backpack size) piping cool air down your back to bring down core temperature. I'm not going to bother digging up a link, but such things exist, some of my less heat-tolerant motorcycle buddies have them. One system I know of has a cooling unit that one would have to strap to the rear seat (probably six-pack cooler sized), but I believe self-contained, don't-have-to-drag-a-box-with-me units are available now.