2018 LTZ has a 6-speed? My neighbor has a 2018 GMC Sierra that's not a Denali and has the 8-speed. OP, are you sure your info is correct?
I have a truck with the 10-speed. I use the parking brake daily. My driveway has a 8% slope, according to the DIC. I use the parking brake to prevent tension on my trans. It doesn't matter the car or transmission, I'd do this anyways. I have no issues with the parking brake. I've noticed that if I forget to release the brake (likely because my kids are driving me crazy) that the brake releases on its own automatically. No issues, tension, pressure, etc. And, if I forget to set the brake while in my driveway and turn the truck off the brake sets automatically. I also have no issues shifting straight from P to D. No cycling through the gears.
I also have no issues with the auto start/stop. I leave it turned on. No funny business going on here with my truck. I have 20,8xx miles on the odometer right now.
Maybe I'm one of the lucky ones. Somehow I haven't had to deal with valve springs, leaky rear window, nor any other issues with my truck.
As for a previous comment complaining about not having puddle lamps, certain seat features, power folding mirrors, etc. It sounds like they didn't get a truck with these features. All are available, and GM's seat functions really haven't changed in about 30 years. Same switches, buttons, or levers with the same functions as always. GM just makes you pay to play.
I am a little disappointed in the stretch belt GM chose to go with. I was just looking at it the other day and noticed mine is rubbing up against something on one side. The signs of wear are showing on that edge. I'm going to have the dealer take a look next oil change. I need to buy a spare belt anyways just in case one breaks while I'm on the road in the middle of nowhere. I'll stick it with the spare wipers under the rear seat.