There was one unusual event you reported in my opinion. You stated when the heater is first enabled it lowers the temperature read. Then shortly after with the heater still running the pre heater engagement temperature is returned to.

This to me indicates the new thermostat more than anything is causing the issue you are seeing. Think of it this way. You turn the heater on. This scavenges heat from the engine coolant in the engine. The thermostat shuts down to restore the heat so the temperature rises until the thermostat opens again.

Since you purchased a thermal reader check the operating temperature in the vicinity of the thermostat housing. With any flow issue or inneficiency the temperature would be held down with you continiously extracting heat from the heater core. Making the temperature remain lower .

Far too many site users have experienced using our heater cores to pull the temperature down when say having a bad radiator in a car or perhaps low coolant level but usually with low coolant level the heater core will not produce heat. Since these cars have a separate heater flow set up they might be differant.

You may want to seriously elevate the front of this car. Take off the upper rad hose and see if you can burp out some trapped air. This can have effects as well. Top off with coolant at that point if the level drops in the hose. This is the only way to properly fill these systems anyways with some assurance all the air is out. Trapped air can impact internal flow patterns inside the engine block and produce diffferent temperatures in different areas than the manufacturer intended by design. My last thought is perhaps a collapsing suction hose on the waterpump. Just feel it to make sure it is not gone far too soft with age.