I used the method where the GX is jacked up in the air (wheels removed and off the ground), the engine is started, a helper runs the trans thu the gears, and I collected around ~2 quarts from the cooler in line in front of the radiator, prior to turning the engine off. Then I added in 2 fresh quarts (like you did), and repeated the process. After ~15 quarts you'll get fresh fluid coming out the cooler-in line in front of the radiator. This is after dropping/cleaning the pan, replacing the strainer/filter, and adding ~4 quarts of fresh fluid to refill the pan.
This method replace essentially all of the fluid with very little opportunities for contamination. The transmission sucks in fresh fluid out of the pan while discharging old fluid towards the cooler (which you intercept and put into a container).
