Based on your prior temps, I suspect the thermostat isn't opening fully. The amount of water flowing through between half open and fully open is significant, given the small area of the thermostat housing.
In a pinch, you can reuse the thermostat gasket, if it isn't broken.
So, just having gone through all this, I'll say:
1. The thermostat STARTS to open at 185°F, ±15°F. It FULLY opens (at least my tested thermostats did) at 210°.
2. I've emptied and filled a number of 80 radiators, of various makes, a further number of times, and never gotten an air bubble in...