You're not answering the question; whether or not you're using G05 or G48 in your system. You're just answering with "i'm using Mercedes glycol." OK, is that "Mercedes glycol" for a 2019 engine? Because it won't be the right one. So, once again, are you using G05 or G48?LCG -
On an 80F-90F day around town, the coupe's M104 runs between the 80-100C range as you've mentioned. Mostly around the 100C line with the A/C roaring on a 90F day. Sure, hit the 85F scale on the thumbwheel, (heater on high) and the temp needle drops fast. However, hit the mountain passes and were running way close to hot, hot for my taste. There's no cooling system headroom here at altitude.
P.S., Yes, yes. Running a 50/50 mix of M-B glycol and distilled H2O is the way to roll.
P.S.S., It's said,, neg -3% specific output (HP/TQ) for every 1,000Ft elevation above sea level. 8,200Ft elevation at our garage - neg 24% output yikes!
Turn your heater on to maximum heat and highest fan speeds if your temps are approaching 120*F. If that doesn't lower the temps you can dilute your coolant mix to have more water in it. Water has better heat dissipation properties than coolant/ anti freeze. That worked for me in Los Angeles, where I drove to Phoenix regularly. Be careful to correct the mix for winter.
If that doesn't do it, you probably have leaves/ debris between the ac condenser and radiator, blocking your airflow. A cooling system in good nick shouldn't struggle to dissipate heat at high altitude. My 2.6 got over the passes just fine, albeit in 3rd gear. And I was hauling ass! Temps didn't get that high.