I like to add semolina flour to my dough, and I also add a bit of vital wheat gluten, but only a couple of tablespoons for 5 to 6 cups of regular flour. The last dough I made with one part semolina to four parts bread flour, but I think you could increase that. Semolina flour gives it a chewier texture, and so if you want it soft inside, you might not like that. I think also that to have it crisp on the outside and soft inside you need to have a good proportion of water. When I bake bread, I spray the loaves with water to make the crust crispier and have a pan of water in the bottom of the oven, but I don't know if that works for pizza, since the topping already has a lot of water.
What oven temperature are you using?