Did you end up putting insulation on top of your oven ? If not, that would help you a lot.
The suggention here also was that the 2 inch fermicrete floor you have would not be to good an isolator. Next time you fire up your oven, check under the oven how warm it is getting. It should not be getting warm... Even though 2 inch is not a whole lot, it is insulation so it should help somewhat. If you add some ceramic fiber on top of the oven (3 inch preferable) I would think your oven should do a lot better.
It will not help you with the cooling down floor after baking a pizza though. This happens with all ovens, even with the best isolated ones (allthough they might suffer from it less than yours). Your oven is probably to small to be baking the 2nd pizza at a different place ? That means you just have to wait a little. When you cook a pizza, it will "take away heat from the floor" and in that spot your floor will cool down. You have to wait a little so the heat will get back into it (your fire will help with that but also the stored heat in the stones will spread and make the cold spot hotter)
You do want fire in your oven while you are baking ! Hot ambers is not enough. You do not want a big fire, just some flames licking the ceiling.
Last problem I see wich you can not do anything about anymore, you used 2 rows of bricks for the "roof" ? If so, you created a lot of thermal mass for your oven. this is great for keeping heat in (if you insulate) but it will take a long time to get hot. Great for baking bread the next day though, you should give it a try
I had a very simular oven like yours but with insulation on top and I was able to cook pizza in under 2 minutes. That were the first ones going in though. The pizza party's we had with 30 to 40 pizza's, the last pizza's always took more like 3 to 4 minutes.