It's too bad you didn't post any images of your new oven, I for one would love to see it ! and I'm am sure many others would love to see your oven also.
How long did you bake the first pie for ? - you say you did it at 420 degrees *C*, which according to http://www.onlineconversion.com/temperature.htm
is a whopping 788 degrees, so I'm not surprised that the bottom is burnt like that.
How did you take the temp of your new oven ? did you use one of these IR thermometers and take the temp from the surface ?
The bottom is extremely burnt, but the top is not, so that would tell me that perhaps the oven was not at that temp long enough,
or something else is coming into play here. How long did you have the oven going before you put in your first pizza ?
One other thing is the sugar content in the dough - if yours was very low, then this would cause the pizza to stay whiter, longer, and you if you were really looking at the crust area of the pizza to brown, then this would have been quite longer in time, and this allowed the bottom to burn.
I see the pizza was on a metal tray in your images.... but was this just for slicing the pizza ? or did you use this in the oven ? ... I'm guessing from the extremely black areas, the pizza was directly on the stones in the oven, and you put it on the tray after ?
On your 2nd pizza that was done at 350 degrees C ( 662 degrees F ) you can see the temp in the oven was much more even, because the bottom is not burnt like the first one, and this tells me the upper part of the oven was now getting nearer to the bottom area of the oven ( will never be the same but closer in temps ) and this resulted in a more even bake on the entire pizza.
Lots of questions, and I'm no expert on authentic wood-burning ovens, but it's a great topic, and I'd like to see more discussion on this. I would love to own a wood fired oven, but I just can't justify the cost, because I just don't make pizza every day, or every other day, but it would be a great toy to own, and I'm sure once you have your methods correct, you will be making some amazing pizzas !