Engine wear is minimized around 80-85F. Maybe a bit higher with synthetics. I've seen people running cool thermostats, and it's always a mistake. I'm not even sure where you would buy such a thing.

Pure glycol is also a mistake, but it's one virtue is that you'll see much lower system pressures.

Cooling systems are pretty simple, but it can be hard to pinpoint a problem. Overheating at speed usually indicates a clogged radiator, the question is whether the air or the the water side is clogged. I'd start with a flush and a new thermostat (stock temperature). Also peel off the aux fans from the front of the radiator and clean out the inevitable collection of dead bugs.

You can test for combustion gas in the coolant with a chemical test kit if there's other reason to suspect a headgasket. Other reasons would be things like sludge in the coolant or sump, evidence of oil contamination in the header tank, or disappearing coolant.