Comment by reddit_clone

Both excellent points, and I would add a recommendation for high level organization: to consider the data then the process. If you can draw a graph where every data structure is only connected to a process, and vice versa, and that processes may take multiple inputs but only produce a single output type, it will make holding the entire system in your head a lot easier, even for very scaled-up systems.

Bonus points if you can distinguish between essential state and circumstantial state.