Gene, you are officially the first person to be working with steel plate along with an oven who's dial only goes to 500. The fact that you're able to hit 530 is encouraging, but you're still a bit behind the eight ball as far as getting the oven temp you'll need for the perfect bake times.
Encouraging faster browning with a bit more sugar would help. You could add more sugar to the recipe, but I'd rather see you bump up the sugar by going with the 2 day ferment that's in the original recipe. That will give you a faster bake time and a little more charring.
When it comes both to launching pizza comfortably and enjoying the best aspects of a NY slice, your hearth can't be too big. As I've said elsewhere, you want the plate sized so that it will touch the back wall and the door will have just enough clearance to close. It sounds like 16" x 16" is what you're going to be working with for the foreseeable future, but, remember, bigger is always better.
And speaking of bigger... for an oven with a 500 dial, 3/4" steel would guarantee you those magical 4 minute bakes. Most people steer clear of 3/4" because of the hassle of reinforcing a shelf to handle the extra weight, but most people don't have your DIY skills
1/2" steel plate will pre-heat to your oven's peak temp (530) in no more than hour. Any more pre-heat than that and you're just driving up your utility bill. And ramping up the surface temp of the stone with the broiler isn't buying you much. Since the thermostat at the top of the oven will always be turning the broiler on and off, you'll never be able to drive the core temp of the plate that much higher than what you get with the hour-ish bottom element pre-heat. The broiler element's most important role is both being close enough to the pizza to brown it quickly and to stay on during the bake.
If you're following my recipe and have the broiler about 7" from the steel plate, I'd move it up a groove.
The extra sugar from the cold ferment isn't going to get you into the ideal 4 minute realm, so we'll probably have to try a few other things, but, for now, since your pizza turned out so well, I want to take small steps. Next time, go with a 2 day cold ferment and move the plate closer to the broiler.