Drove around a little bit without the thermostat. Seems to work just fine. Maintains its temperature doing 45 with the fan off, at around 180.

I picked up a cheap 180 and 195 thermostat and just boiled both. The 180 opens 1/4"+, and the 195 just barely cracks open like the other two I have

I'll see what the flow is with the 180 installed, and if I feel lucky I'll swap the new(est) 195 in and check the flow again.

(Final?) update. I installed the 180, and at 200 it's really moving water through the radiator.

I need some additional logic in the fan controller to turn the fan off if the temp goes too low, but otherwise, it was able to keep the temperatures around the set temp of 205.

So, it appears that my issues were based around the 195 thermostats that didn't open enough.