When driving on the highway are you getting adequate heat in the passenger compartment? If you aren't or weren't then your thermostat may have been stuck open or partly open. That is of course unlikely with a new thermostat in place.

If your heat is ok on the highway the engine temp is probably ok at that time.

I would feel the upper and lower radiator hoses with the engine thoroughly warmed up. The lower one will not be as hot but if it is very much cooler than the top it seems likely the coolant is not circulating well through the radiator and the radiator would need cleaning or replacement. That in my experience can lead to overheating either in the city or under load (such as when taking a long hill).

The ECT sensor controls the temp gauge and if bad would lead to incorrect gauge reading even with the actual engine temp ok, but you would get a diagnostic code if it were very far off, and the car would run too rich or too lean. Anyway you replaced it so it should not be at issue.

I believe there is a sensor on the radiator which controls the fan through the relay. Is that sensor stuck? If stuck on the car will run too cool on the highway and warm up slowly from a cold start, if stuck off the engine will run too hot unless you are moving at speed.