Paulo, well done.
For what it's worth, the heat balance gains of my broilerless design, were, up until now, theoretical. The theory was/is, imo, rock solid, but, thanks to you and your perseverance, we now have real world results.
We've got some ground to cover, so, let's get to it.
As I mentioned on the broilerless page that I linked to, an Infrared thermometer is critical for this setup. The only way to know what your oven is capable of is to take plenty of temperature readings. Here are my recommendations:
Dealextreme is very competitively priced, but it ships from Hong Kong, which takes a while.
It could be the photo, but a piece of the quarry tile looks like it's raised a bit, creating a lip to your hearth surface. This can cause tearing. Are your tiles flat?
I'm not seeing enough flop, which is a bake time/water thing. This is going to make your present bake time issue even worse, but we need to increase the hydration. I think 66% is in order. That should compensate for elevation a bit better. A thinner stretch will help compensate for the longer bake time from the additional water. I think you've made enough pizzas to start venturing into more difficult stretching territory. Next TF: .075.
Speaking of compensating for the water and decreasing the bake time, we're ready for 2% sugar. That's as high as I'd recommend, though- enough to achieve browning, but not enough to create any kind of perceived sweetness.
The texture looks fantastic. So good, in fact, that I'm going recommend adding oil again. Oil is a browning accelerator, so, just like the sugar is helping out in your slightly cool environment, oil is important as well. Next: 2% vegetable oil.
That's about as far as we can take the formula towards encouraging both floppiness and browning. Now on to the oven setup.
It looks like the quarry tiles that you're using lack enough conductivity and are thick enough that they might not need deflection. Lose the pan underneath.
You mentioned uneven browning. You're using a metal turning peel and turning the pizza during the bake, right? Also, related to browning- I can't tell from the photo, but is there a gap between the tiles and the back wall? The tiles need to be centered, front to back.
The position of your thermostat probe (center top) is a bit disappointing. I think upper corner is a more common placement. The fact that your probe is right over your ceiling gap is allowing the rising heat to drive the temp up quickly. Here's my suggestion. Take the pan that was hanging under the tiles and put it, with some kind of spacer, on top of the ceiling/across the gap. Do you have any stray bits of tile around? 4 small pieces (anything from the size of a quarter to the diameter of a lemon would work) would work nicely as spacers. The goal is to lift the pan, so that hot air can come up the gap and then flow up and around the pan. Because it's moving the heat to the sides, your thermostat should heat up a bit slower.
I'm curious, can your thermostat be unhooked/unscrewed and moved elsewhere?
I mixed for 5 minutes with the paddle attachment, and another 4 with the dough hook. Finished dough temperature was 71 F. Do you have any recommendations on using the paddle and/or the dough hook when mixing, or if my mixing times seem a little lenghty?
The mixing/kneading time is always based upon final appearance. Based on the photo you have here, and the assumption that this is 13.6% protein flour, it's a bit long. Next time, try 4 minutes paddle, 3 dough hook.
As you're fully aware, you're moving in the right direction. Let's tweak the formula, order a thermometer, see what hearth temps/ceiling temps we're achieving, and start thinking a bit more about probe isolation.