Your crust is probably coming out on the dry side because you're par-baking the crust, pulling it out, then baking it again. Since you raise the temperature of the crust during your first bake, the inertia of high water vaporization has been overcome and you will begin to lose water during every moment you have the crust out of the oven, in addition to when you have it in the oven. Add up all the minutes it takes you to bake the pizza from the time you put the pizza in the first time, to the time you pull it out the last time. It's almost as if you're baking it for that total time since you'll be losing water out of the oven infinitely faster than the crust is baking, and assuming the crust reaches 140 F, 6.5 times faster than at room temperature.
EDIT: I don't really have much more advice on baking surfaces. If you had an unlimited budget, we could talk about your options again, for there are several that I didn't mention. You can try cast iron to see if it gives you the results you're looking for. However, I still don't know why you think the stone needs to be hotter though. If the stone is 600 F, and you're baking your pizzas at 550 F, you're already providing a decent head-start to the bottom. Moving the stone closer to the bottom of the oven provides an even greater advantage. Setting the stone near the broiler to preheat while leaving the oven door cracked open should get the stone super hot, and I'd say that's about as good as you're going to get without exploring severely expensive options.