Is this the hygluten version of the Sperry or the regular?
There's a variety of substitutions for brick cheese, and the direction they like to take is cheddar-y. Personally, I've never worked with brick cheese, but, from the way it bakes up, it looks more like a quality mozzarella than cheddar. I do know that provel is not a part of the equation- unless you've spent time in S. Louis and that's your preference. I think you'll see a much better melt with mozzarella- if not 70/30 motz/jack, then 100% of a good aged mozzarella- look for yellow and firm, not white and soft- wholesale is ideal.
The sauce for Detroit is always cooked separately and added post bake.
I played around with Detroit for the first time this week (sshh... don't tell anyone). One conclusion that I came to is that if you add oil to your dough, it acts like a magnet and really ramps up the oiliness in the finished product.
Detroit is typically not parbaked. I think you figured this out by the fact that you needed to broil it. Getting rid of the parbake will go a long way towards giving you a better cheese melt, as the rising steam from the dough as it cooks will help bubble and oil the cheese off.
There's going to a be an oven shelf where the top and the bottom finish baking at the same time, but, until you figure it out, I'd go with the lower middle shelf, and start checking the color on the bottom after about 12 minutes. If the cheese starts taking on too much color, until you get the right shelf, you can slow down the cheese with a misting of water.
Is 500 as hot as your oven will go?
I preach quite a bit about the evils of excess water in pizza dough, but, for Detroit, there's a practical aspect regarding the water. Lower water doughs are going to take considerably more effort to stretch into the corners of the pan. Depending on which Sperry this is, I might kick the water up to 70%- or possibly even higher.
Kenji does a thing on his pan pies where he gently lifts the dough just prior to topping so that the bubbles between the pan and the dough deflate.
Edit: Oh, and I'm sure you're working towards this, and getting rid of the provel will help, but, it's essential that you build your cheese against the wall of the pan, so it fries and you end up with the characteristic 'frico' of Detroit. Get your frico on :)