The problem here is simple, and Toyota has identified it. It is simply owners who do not use good oils and go too long between changes.
Sludge is ONLY formed when using PETROLEUM oils. At high heat, petroleum oil forms sludge.
Solution? Use ONLY a PREMIUM 100% synthetic Oil. No more sludge.
As far as the other ideas suggested like changing to a cooler thermostat, etc. DON'T DO IT!! That engine and computer is designed to run at a certain temperature. And, putting in a cooler thermostat can actually cause your engine to run hotter, just like having no thermostat.
A thermostat is designed to keep the coolant in the radiator for a certain period of time. Then at the desired temp it opens and the coolant flows. Opening too soon due to a lower temperature can cause the coolant to flow out too quickly and not have the time in front of the airflow needed to cool.
Every engine is designed to run at a specific temperature.