If your thumb is pointing down, it is not a Goon stroke or a stern pry. The Goon stroke/river J/stern pry is executed with the thumb up and is sometimes called a “thumbs up” J stroke.
What you are doing sounds like what I would call a stern rudder, which could be done with the thumb up or down. It is true that the further out to the stern you can get your paddle blade, the more effective the levering force of a stern rudder or stern pry will be. The problem is, if you apply power on the blade all the way to the stern of the boat, you will be pulling the boat down into the water on much of your power phase. You can reduce that tendency by feathering the blade where you would normally end your power stroke, and keep the blade feathered all the way to the stern of the boat.
This works very well for directional control but results in a long, slow stroke that kills paddle cadence, as has already been mentioned.
Although the J stroke is smoother than the “thumbs-up J” power stroke/stern pry combo, I know some flat water paddlers who routinely use the latter in preference to the J, and they seem to give up very little efficiency doing so.