Just an update on how this turned out: I replaced the broken thermostat and tried it out with the old radiator. It did not really seem to make a difference (maybe a little but not definitive). I then put in the gemini radiator and it now seems all good.

I was surprised the car was only running a bit hot (2/3) with a physically broken thermostat that was decidedly stuck closed, and that replacing the part did not seem to make a difference.