Hi Omaha, and welcome to the forum.
I can see you've read up on the causes of the "dreaded gumline" because you have already preemptively stated that you are doing most of the things you can to prevent gumline. If despite all of these precautions you are still getting excessive gumline I would think it is because your gluten structure in your dough is not sufficient. If you think about it, the gumline is just a part of the dough that doesn't rise sufficiently. It's a part of the dough that is not properly leavened to combat the moisture from the sauce layer so it just sits there flat and doughy instead of puffing up into delicious pizza crust. So maybe you should review your mixing technique to make sure you are getting sufficient gluten development in your dough. With better gluten development you your dough should be able to stand up better to the sauce and puff up when the heat hits the dough.
When do you add your salt to your dough? If you add it at the outset with the flour and water, maybe you can try holding the salt out until the dough has mixed for a while. That will help your gluten to develop more. Also, do you autolyse (i.e., hydrate the flour and and let it sit for 20 minutes before mixing the dough)? If not, that can really help with gluten development.
I assume you are using the same basic ingredients as the bakery that used to make the dough, i.e., same flour, same yeast, same water, correct? If not, then your problem may be in the ingredients.
The other thing I thought of was whether it might be possible to go back to the bakery which used to make your dough and ask them for help. They are perhaps not too happy with you for cutting them out of the profits of selling you dough but if you can, maybe you can go over your procedures with then to see what you might be doing differently. Also, consider how the trip from the bakery to your shop might have affected things and what you could do to emulate that trip in your process.
I should stress that I am a total amateur and have never owned a pizza shop (though I did work at one when I was in high school). My comments are just based on common sense and my years of making pizza dough at home. Hopefully Tom L. will weigh in and give you more professional advice.