For Neapolitan, 13-14" is just the right size, with larger sizes feeling strange, imo. For NY, though, the bigger the better.
This pie is definitely a step in the right direction.
Was this another hour pre-heat on the stone? Again, we need to see what malted flour can do, but, for unmalted flour, the bottom is still looking kind of anemic.
While I did say that broiler pre-heats are superficial/ineffective, you did seem to be getting a net increase of around 15 degrees, so, based upon a comparison of this undercrust to the last, I'm not entirely you should get rid of the broiler pre-heat or move the stone to a lower shelf, which is going to really impact any broiler pre-heat adversely, because of the increased distance from the IR.
If the top is browning too quickly, don't move the shelf down, just turn the broiler on for only part of the bake. Out of my 4 minute bake, I only use the broiler from minute 2 to minute 3.
While I think that getting pro flour is important, I think it's even more important to dial in your oven setup and make sure the stone is browning the pizza in the right amount of time. For this reason, I would, while you're looking for pro flour, get a bag of better for bread just to test the properties of the stone. That way, if this stone can't give you half decent char in 4 minutes with a malted flour, you can start stone shopping as well.