Yes, 530 is a safe temp to make sure the broiler will stay on during the bake, but I think with your lack of undercrust browning, you need to push the envelope a bit. Pre-heat the stone on 550 for 90 minutes, turn the oven off for 3 minutes, then launch.
This might rob you a little bit of some broiler time, but you should still see decent top browning in 5 minutes with considerably more undercrust color.
Are you using the broil feature during the entire bake? Is the broiler staying on the whole time?
8" is the second from the top shelf, right? How far is the steel from the broiler on the top shelf? Can you achieve a 5-6" distance? If so, then you'll be able to use your broiler a bit less, so, if it cuts out,it won't be the end of the world.
How much sugar and oil is in your recipe?