For that thickness of brick, unless you're working with an abnormally high BTU oven, you're talking a 2 hour pre-heat, and, even then, with the conductivity of the brick, it's going to be a long lifeless bake.
500 is a pretty major handicap when it comes to home ovens.
If you've got the money to gamble, you could try 3/4" silicon carbide. It's unproven, but it might give you respectable bake times at 500 because of it's high conductivity.
Otherwise, it's time to think about some sort of oven mod. You can go in two directions.
1. A gentle oven mod (a 50 degree or so bump), with 1/2" steel plate as a hearth.
2. A less gentle oven mod (150 degrees or so), with a lower conductivity material such as quarry tile. I know you already have the firebrick, and that would probably work alright in the 650-700 realm, but it's so thick, your pre-heat times will be through the roof.
My vote is #1. A 50 degree bump is very safe/won't put the oven in any jeopardy whatsoever and 550 with 1/2" steel plate is a very proven combo.