But step 1 is exactly the problem. I assume whatever production line makes my frozen pizzas looks pretty much like Zume's robotic pizza assembly line - except for the part where those are frozen and can be shipped from a single factory to a whole country.

Zume reinvents the whole process, but instead of freezing it they put it in a delivery truck and heat it up on the last mile. Which means the factory has to be close to your delivery area. Now, factoring in competition from all other pizza delivery services, how much scale can you achieve with that? How long until automation of these small-scale runs pays off? And how many sufficiently big cities are there to expand to?