>>21454928
OP what you can do (and what I do) is after baking it normally to your liking on the top, move the entire cookie sheet down to sit directly on the floor of the oven and crank the temp to 450+ and let it bake on the deck of your oven for an additional 2-3 minutes (no more than that at a time before you pull it and peek underneath to check the color or you risk burning the bottom). This works because cranking the oven temp causes the heating elements to activate again and there are TWO sets up of them in modern ovens, one visible at the top (the broiler element) and another at the bottom covered by a heat shield and you harness that one's baking power to brown the bottom before the rest of the pizza cooks any further with the heat shield distributing the broiler heat turning it into a giant griddle.

Alternatively you can the entire cookie sheet out of the oven and put on your largest hob on your stove and turn it to medium/medium-high and "saute" the pizza bottom like it's a giant pancake turning it every 15 seconds or so it browns evenly.