Changed oil myself at 3000 miles on my 2018. I got my supplies of 8qts full syn 0/20 (I forget the brand but it was Penz or Valvoline or something decent) and PF63 AC Delco filter at Walmart on special for around $44 for everything. I did not reset the oil change clock because I wanted it to be invisible to the dealer.
Yesterday I got my first of two free maintenance services at the dealer at 7000 miles, which includes oil change, tire rotation, checklist of things. I'll do the second free maintenance at the dealer next spring and probably keep to what GM recommends after that to minimize any warranty issues.
If the truck was out of warranty, I would do whatever I wanted, but until then I'll go by what GM and the dealer says.
As far as the dealer cost for the maintenance service if I had exhausted my freebies, I believe it's around $60. They are always sending coupons, etc. with that price.