since there is not a nevidence of a leak or noise and fans are coming on and the coolant is clean like you said .
1- I would do a bleeding procedure and the coolant system, make sure we have no trapped air
2- I would check my radiator cap , you can test with a coolant tester that is opening and closing correctly
3- check my thermostat for not getting stuck and opening at operating temperature .
4- how is your heat , does the hot air temp increase when u have it on setting of max heat , sometimes if we have a air bubble in the heater core system it will be evident under load .
5- doing a pressure test for checking for a beginning of head gasket issue is a good idea, they can do that by hooking up the tester and let the car run and see if the needle vibrate after the thermostat open and running at operating temperature
6- a weak water pump will show under load ,like going up hill ,or giving the car a kick down
have you tried on level service to give it a kick down and see if the temp needle go up
let me know thanks