Added to better answer the specific questions:
Perhaps This Answer I wrote a few months ago will help; it outlines how I (a discovery writer) approach a story, consistent with the Three Act Structure (3AS) (But I split the second act into two equal parts; 'Reactive' and 'Proactive').
Understanding the 3AS is the answer to: "Is there some sort of rational way to help me understand, and eventually decide, what are the next steps to take?"
YES! As you progress through each half-act (about 1/8 of the book), ensure you have accomplished what that story segment calls for.
As for "When do you decide that it's time to close your story (as a discovery writer)?", the 3AS tells you that too. Each half-act (eight of them) serves a purpose in the story, and you need to progress from one to the next. For example, the first half of Act I introduces the "normal world" for the protagonist, and ends with an "inciting incident" that will eventually drive her (physically or metaphorically or both) out of her normal world (whether she finds it terrible or great). The second half of Act I shows the rising consequences of the inciting incident, and leads to the hero somehow breaking from her normal world to address the issues. And so on; until the second half of Act III, where the hero figures out how to solve the issue, and confronts whatever "villain" she is fighting (it could be a person, or situation, or internal mental issue). As we began with her "normal world," the conclusion may be a return to that, or often is a description of "the new normal", and where she fits in the world now.
So Streamline what you have done; eliminate extraneous prose. Then Map out what you have done, into the 3AS. Each of (I, IIa, IIb, III) should be 1/4 of the book. (If they are not roughly equal lengths; you likely have tangents and details unnecessary to the plot). Where you are tells you what to write next.