I wanted to add my experience changing the fluid on 2018 GMC 8L90
This is in no way to detrack from all the Great information posted by the OP, the information he proved is very good. This is only for additional experiences with changing fluid on our 8L90 transmissions.
Removed the shield and pulled the fill plug. Then used a oil vacuum pump to pull out most of the fluid from the pan with a dip tube inserted in the fill port. Removed the pan bolts which allowed the pan to tip down at the end facing the front of the truck. There was enough clearence to remove the filter with the pan tipped down at the forward end. Then turning the front of the pan towards the front right tire and tipping up the left side of the pan allowed the clearance to move the pan forward and down. I did not need to unplug the connectors to rear solenoids.
Cleaned the pan and gasket. Then installed the pan leaving the front end tipped down, then slid in a new TF922 filter and leveled the pan and reinstalled the bolts. Added 7.4 quarts LV HP Mobil one.
Ran the the truck to bring the fluid temperature to 110 degrees and at that point removed the stand pipe plug from the bottom of the pan. The truck was level, running in park, and on jack stands for this.
The transmissions was about a quart over full and this drained from the stand pipe. Reinstall stand pipe plug, fill pipe rubber plug, and heat shield. Test drove, all was smooth. Its a pretty easy task but I miss the having a dip stick.