Have the radiator temperature read with an infra-red thermometer. That can tell you if the cooling system is getting too hot. It is normal for a cooling system to safely operate at 220F degrees. Yes, after the thermostat opens, the coolant does get hotter than 170F degrees, and its temperature goes to 220F degrees. This is normal. Changing to a cooler thermostat does nothing beneficial. In fact, it can increase carbon deposits in the cylinders and the exhaust.
Make sure that the idle rpm is where it should be. An idle too low can turn the fan too slowly for good cooling.