Make one or two 18" pizzas right before lunch, and, as you sell the slices, gauge the demand so that slice pies are coming out of the oven close to when they're being sold and nothing's sitting around too long. Warm them as people buy them, but if the slice pie is still warm, don't warm them too much.
Topping pre-baked slices is kind of lame. Extra cheese never melts and pepperoni doesn't crisp up the way it should. I would consider just doing plain slices.
18" is a traditional size for a slice pie, but larger slices pies (21") can be really phenomenal.