Turns out I now have a related problem in Windows, with the integrated GPU spinning at full throttle despite not doing anything important.
I've somewhat improved battery life(and CPU temperature) by setting the system to prefer the discrete GPU, which is a ridiculous solution to a problem which I shouldn't have had in the first place.
At this point I think I can live with selecting one of the GPUs and sticking to it for a given session, like I did in Linux on my previous machine. Even if I have to restart the system each time.