Since this is a prototype for the factory shrinking mechanic I will focus on the factories. First it was not immediately obvious how to get back out of a factory. I had to get read the instructions again. Maybe put a button for that somewhere. This experimentation with exiting left me with multiple extra factories. Which leads to my second suggestion. Give an option to delete factories. Maybe limited to only factories that have no instances placed to prevent complications. Being able to rename them from the outside and maybe even sort them would be nice too.
In regards to the prototype itself my main other issue was not having enough belt options. With the belts only being able to split equal amounts in opposite directions it substantially limited how well I could place factories to take advantage of large producers.