I have a 5 of the 2020 Duramax's in my fleet. Yes they loose mileage after the update. Dealer has been of no help
Noisy brakes, Yes there is a dealer bulletin about the brakes. Now getting your dealer to fix it is a joke.
I just ordered new brakes after the dealer commented that GM will not cover it with 50k miles. I have reported it for the a two plus years I've had them.