Thank you both so very much.
Turns out I misstated the temperature in my first post. The gauge is marked with an "80" and a "120", so the unmarked line in between I assume is 100.

It used to run at 80 and go to 90 if stuck in traffic, and then drop fast to 85 (which I'm guessing is because the fans came on).

After the super expensive new thermostat and flushing by the dealer, it now runs normally at 85 and when stuck in traffic stays at 100 (the unmarked line) until I get really moving again. That worries me because it is much higher than before the work was done.