Hey 500. I know how you feel about the heating game.
This is what I have experience being a factory tech.
One time I had a over heater a pretty good one. I had to replace 3 t-stat all defective. What I rec is going to get a new thermostat but ask for a 75 C opening temp they sell them. Also when you get it, on the outer ring looks like check valve almost looks like a ball bearing/get a pair of pliers and remove it you should have a clean hole after that/that lets more coolant/better circulation. An old timer benz tech told me when i start 13 yrs ago. If it doesnt have this checkball looking thing/drill a nice clean hole on the outer ring. Trust me this will help I have a 500E that runs hot all the time and I did all what youve done so far. But rem/ make sure the radiator has suffcient thermal effiency all areas of the rad needs to be hot and up to temp for all fans to work correctly.