So I posted this awhile ago. Sunday I ended up replacing the thermostat and doing a full flush of the radiator and block. Refilled with the correct coolant (previously had the green stuff in it, Mercedes recommends Zerex G-05. Drove it in to work today, whole time on the way in and way back it quickly ran up to ~97C but then stuck there firm.
Just got back, threw it in park and it was still around 97C. Popped the hood, opened up the coolant overflow tank and sat inside with the heat on full blast. Sure enough, temp started to drop all the way down to maybe 85C. So I guess there was just a bunch of air trapped in the system? I figure that sitting in park with no airflow through the radiator and temp already high would be the worst case scenario for it, so if temp was able to drop back down, that’s a positive.
I’ll drive it in to work again the rest of the week and hope it continues to run cool.