I'm away from my home base but I'm fairly certain that the recipe you are using is based on the Tom Lehmann recipe for NY style dough and produces a dough ball having a weight sufficient to make a 16-inch pizza.
You can substitute bread flour in the recipe, as you have done, but it won't produce results as good as high-gluten flour, even if you add vital wheat gluten to increase the protein content. However, should you choose to go that route, for the above recipe you would need about a tablespoon of vital wheat gluten (I use the Arrowhead brand) to increase the protein level of the bread flour to approximate high-gluten flour. It would be a miracle if you could find high-gluten flour at the retail level. Only passionate home pizza makers and professional bakers are interested in high-gluten flour and the professionals can do much better pricewise buying in 50 pound bags from millers or wholesalers rather than 5-pound bags at the retail level. I have tried several brands of high-gluten flour and believe, like many others at this forum, that the King Arthur Sir Lancelot brand is quite possibly the best (but burdened with high shipping costs).
The high-gluten flour will give you slightly more browning than bread flour because of the higher protein level (about 1.5% higher for the KASL brand). As has been suggested, using a stone or tiles (with enough surface area to accommodate a 16-inch pizza) may do a better job of browning than a pizza screen alone. Or you can combine use of a screen and a pizza stone or tiles (e.g., you can bake the pizza on the screen for the most part and slide the pizza onto a stone or tiles for the last couple of minutes or so to get additional bottom browning.)
The Lehmann recipe does not call for added sugar. But if you plan to keep the dough for more than a day or two before using, or if you want additional browning, then Tom L. recommends a level of sugar equal to 1% to 3% by weight of flour. For the above recipe, this would come to 0.12 to 0.35 oz., or about 1 teaspoon to about 1 tablespoon of sugar. The higher amount might produce a sweeter dough and will be detectible if you have a sensitive palate to sugar. I would tend to start with the lower amount (or even lower, as Giovanni recommends) and work up from there based on your results. Or you can follow Steve's recommendation to use the Carnation Original malted milk. I am curious about whether the malted mill will improve upon the Lehmann recipe.
If you propose to use dry milk, unless it is a high heat or baker's grade of dry milk, you may want to first reconstitute it in the water, heat it to scalding and let it cool down before using. (There are things in ordinary dry milk that are said to interfere with the fermentation process).
As for your question about proofing the yeast, no proofing is required if you use the IDY as set forth in the recipe you are using. The IDY should be mixed directly into the flour. If you use ADY, or fresh cake yeast, and make the necessary quantitative conversions from IDY to ADY or cake yeast, I recommend that you proof the ADY or cake yeast in a small amount of the water (warm), and combine with the remaining water after the proofing is complete (about 8-10 minutes). If you are temperature adjusting the water to achieve a desired finished dough temperature, the temperature adjusting would be with respect to the remaining water.
Good luck and let us know whether you get better results.