Before changing things willy-nilly .. how about a simple check

Start engine from cold .. leave to idle whilst standing with one hand on the big pipe that goes from the thermostat to the top right (as you face the engine) of the radiator.
One of 2 things will happen ... EITHER the pipe will slowly start to warm up and never get really hot .. OR the pipe will stay cold until a sudden rush of fairly hot water passes through (there is a 3rd possibility but that is linked to overheating so does not apply here.....

)
If the pipe very slowly heats up .. the thermostat has failed open .. it needs replacing.
If the water stays cold until a sudden surge of fairly hot water .. the thermostat is working .... in which case it is probably an indication problem.
It is, rarely though, possible for the thermostat to work but at too cool a temperature, in this case the "surge" of heat will be mush cooler than "fairly hot" .. if that makes sense ..

HTH