The correct pressure in the system is important for the cooling system to transfer heat out of the motor to the radiator and into the air. I just can’t understand your symptoms. Are you just way to focused on the temperature guage? Or, is there a real problem?
After all this work could a hose be pinched and not be allowing good coolant flow? Has all the air been properly bled from the system. What is happening with the overflow coolant tank. Does the fluid in there expand so the there is more fluid in the tank and then does it go down to a lower level in the morning before you start the car for the day? The overflow tank should be about 1/3 full when the motor is cold, say 2" in the bottom of the tank. When the motor is hot is should be about 1/2 to 2/3 full but still have about 2" of air at the top.
If the tank is empty in the morning when the car is cold then add a 50/50 mix of coolant and water, but don’t fill it more than 1/2 way. Run the car and check it again the next morning. Add coolant mix again if it is empty. Keep adding day by day until the tank is stable when you check it cold in the morning at the 1/3 level. This will get rid of any air that may be trapped in your system. Air in the cooling system can cause some crazy symptoms and may contribute to some of the stuff you are seeing.