Hello Hunter and welcome to the Brickwood forums!
[Edit: I got interrupted while I was typing this, and posted it before I saw that @BrickWood had posted a much more detailed response! And, as always, I learned something helpful from his post.]
I like what you’re planning. I’m not sure I’d want to simply double up on firebrick.
One of the jobs that the insulating layer under the hearth accomplishes is providing a “break” between the cold concrete slab and the very hot environment in your oven. The firebrick absorb heat energy, but they eventually pass it through. That’s the same reason you have mineral fiber insulation around the arch of the oven, rather than extra layers of firebrick.
It’s important to keep your floor insulated because you want that sizzling 800°F+ heat underneath a pizza crust—or otherwise why bother building?
It’s totally doable to build a frame to suspend your void form on top of your concrete pour. I’ll leave the details to others who have experience with it.
I am so eager to see photos of your progress and the final outcome. Glad you’re here!