Having been a prior owner or a freshly redesigned vehicle in the past, I would strongly discourage purchasing a first year redesign vehicle. If you can wait for the kinks to be worked out, then go for it and wait. Just keep in mind if you don't like it, it is highly unlikely you will be able to get a left over 2021 equipped the way you want it. Hopefully the window issue is corrected now production was moved back to the states rather than Mexico. Time will tell. I have a 7 year warranty.
As for advantages of the 2021, the 6.2l moves, has an excellent drive, and is capable of just about anything you throw at it. I don't know why everyone Knocks the interior. Everything is placed in the right spot, interior room is great, and esthetically it is not too busy. Take a look at the tahoe or yukon for an idea of what the interior of the 2022 will look like. See if you like that better, as I will place money the sierra/ tahoe will look similar if not identical. If you haven't already, take a 2021 for a test drive and then make the decision.
Good luck OP, it's all really a preference, and everyone will have an opinion.