The thermostat appeared to function normally.
Looks like the temperature sensor is out of spec. Iím getting a reading of 1.2ohms and the spec is 2.1-2.6. It says to replace if itís off by more than 10%. Iím going to run it through the temp range in a pan of water to be sure.
I'm beginning to get a sense that maybe your cooling system is actually OK and the gauge is just getting spooked by the sensor.
(Keep in mind that if one has a fully flooded cooling system with a properly operating water pump, thermostat, rad and for sure has system flow, then there is no cooling system problem per se.)
Your stat' check indicates no fault re that component.
Be sure to make a point of doing the sensor ohms range check all the way to rip roaring boil on the stove.
If the Ohmage range is duff, sure, go ahead and get a new one, and hopefully that solves the problem.
You have 85% + water, right?
For the cooling system cap to release, seeing as the cap is a 16 -20 psi rating, and there is still some EG in coolant, a coolant temp in excess of 250 F would be required.
I doubt a heat soak after a max engine case and engine oil temp run would get to 250 F, so I doubt the rad cap will be of valid insight, assuming the cap is still within spec.
By the way, referring to your gauge picture, I once got my gauge just about there too, on a track day in very hot conditions, with thin dry 3500 ft. altitude air, 75% water/25 % EG/WaterWetter.
The main straight was over a half mile long and the bike was hitting 135 mph before braking, so very high heat load for a fair bit of duration.
I didn't realize that I should have removed the fan for such a track and day, as the fan acts to reduce the effective rad area at high speeds.