High-Level Prototype Requirements
Now that you've split your product into different components, we need to determine what to prioritize for the first release.
Modern development is done in small, repeated cycles, called Sprints or Iterations. Sprints are supposed to end with something presentable to stakeholders. You can't do that if you have no UI. A UI that does nothing when you interact with it is useless.
What are the minimum backend and frontend functionalities required to have the project compile? All you want is something that kind of looks like what they want in the frontend and minimally performs in the backend.
This is the most important goal to start with, and after it's done the whole project will flow much smoother because it's just adding small features one at a time.
Break these basics down into goals-which can be abstract for now-and put them on a shared project board.