Bill, if you alter your approach, pre-shredded cheese will work just fine.
1. Make sure the pre-shredded cheese is whole milk, not part skim and not a blend. The milkfat goes a long way in helping the cheese melt and with less protein, the cheese browns more slowly.
2. Standardize the consistency of the sauce and, if you're not already, start measuring it. Your cheese, as it stands now, is just melting and then browning. Pizza cheese should bubble and boil. In order to bubble, it needs to have sufficient moisture below it. That means a sauce that's got enough water and is in sufficient quantities. I work with a can of 28 oz. puree that will almost form a peak when spooned, to which I add 2 oz. water. I also use 7/8 C. sauce for a 16" pie. You want to be careful not to add to much water or the taste will suffer and the cheese will have a tendency to slide, but you need more water and more sauce than what you're using now.
3. In order for cheese to bubble and boil, it has to get heat from below. A pizza screen is, as I've mentioned before, the kiss of death for fast bottom browning. In order to get the cheese bubbling, you need a stronger blast of heat from the stone. To achieve this, you need a more conductive and/or thicker stone and/or hotter oven. Also, it's a little hard to tell from the photo, but you might want to look at your thickness factor. Less dough = less insulation underneath the cheese = better bubbling.
4. More fat helps. I use a part skim/whole milk blend, that, if I bake it without pepperoni, it will brown, but with the fat from the pepperoni, it bubbles beautifully. I've never tried it, but a sprinkle of EVOO might help. A light spray of water on top of the cheese (avoiding the rim), also helps the cheese bubble a bit easier, but you want to be careful, as too much water will have a tendency to promote curdling and cause the cheese layer and sauce layer to combine.