I think what you are seeing is a radiator needs to be removed and either thoroughly cleaned or replaced. The 05 Duramax I had did the same thing. Coolant temps went up & tranny temps were up also. I replaced the radiator and everything went back to normal. I expected build up inside. But was quite surprised at the amount of trash covering the face that had gone thru the rest of the stack. It’s a 15 year old truck, the radiator isn’t as efficient as it was. It’s now borderline for the load your putting on it.