I once learned this sugyah with a prominent Rav in Brooklyn. Here are the issues:
When something is made with flour, it depends on what else goes into it. If it’s made as a standard bread, it always requires a hamotzi. If it has ingredients to give it a flavor other than standard bread (even if it’s not mixed directly into the dough), it’s called pas habah b’kisnin. Pas habah b’kisnin does not require a hamotzi unless you’re koveia seudah on it. Pizza, because it has sauce and cheese on it, falls into this category. That is why many people say that you only need to make hamotzi if you have 2 slices or more. However, that refers to fresh pizza. Many frozen pizzas, or pizza bagels, are made differently. They make the dough and bake it without anything else on it. They then put on sauce and cheese and immediately flash-freeze it. In such a case, it’s not pas habah b’kisnin, as the bread was baked without anything else on it. Once it’s considered bread, it doesn’t change status easily (there are some ways it can, but the pizza isn’t one of them.)
Therefore, having one slice, if you’re not koveia seudah on it, does not require a hamotzi.