Zero issue with going under the car with the quick jack. It has a manual lock that keeps it secure, care is very stable. Personally feel much safer under it than jack stands.
And why is that? So you’re advice is to not follow the GM TSB? And without a flush you will not get out all of the old oil to replace with the new recommended oil.
Get it done now...if your car is under warranty. He bulletin states the dealer does not have to verify the complaint. The customers word is enough to get the flush done.