Thicker tomato sauce on the pizza makes the cheese more slippery
If you have to trowel the sauce on the crust it is too thick.  There is a "right" consistency that will prevent the pizza from being too watery but will still have enough moisture to allow the cheese and sauce to fuse somewhat.

On the other hand, you could just ignore that the cheese isn't fusing.  I've eaten thousands of pizzas in Italy and most of them are more watery that what you'd get in the US and the cheese usually does not fuse with the sauce.  Probably because it's only in the 800°F brick oven for about 3 minutes!  Enough time to cook the thin crust, barely melt the cheese, and that's it.

If you really want that cheese to fuse, I would suggest a slightly more watery sauce, and more heat from the top.  Turn on the broiler for the last minute or so until the cheese starts to bubble.  This will also help evaporate some of the extra water in the sauce.