Komi,
Your project seems to be suffering form the classic problems that plague any creative effort like a book, or movie, or software. One other thing using a tool to organize your work will do is let you better visualize how long it will take to get stuff done. Skip this if you've ever worked on a scrum team or other agile team - I'm writing all of this because I get the impression that you haven't - but basically, if you want to get stuff done on time you need to A) know how much stuff there is, and B) have a good guess about how long it will take to get stuff done. Organizing your work into time-boxed periods (sprints) where you commit to getting a certain amount of work done in that time, then constantly reviewing and refining your outstanding work items and estimates, lets you visualize your schedule. To run a good sprint, you need to have a plan for the sprint that is clear, and the ability to complete the work you commit to in the sprint. Sprints are short, like 2 weeks. After each sprint, you review what went well, what didn't, and how good your estimates were, and you eventually get a good sense of how much work can be done in a sprint.
Once you know how long the upcoming work will take, you will be able to guess how far your schedule slips if you add new work. Nothing will ever be released if you never stop changing it, so you have to, at some point, stop adding new things.
Not adding new things means maybe all of Chapter 1 doesn't get some cool new feature throughout its story, but then, that's functionally equivalent to "I'll add it in Chapter 1 release 3" or whatever.
A software team typically has changing requirements and scope creep all the time, and it takes project management and editorial skills to push back on that and to cut things that aren't necessary if you want to meet a deadline. Of course, sometimes it's worth slipping a deadline if it means a better product, but a later product might not be a better product either... just look at CDPR's latest releases, where they've had missed deadlines AND shoddy releases. I have the impression that you're struggling with both sides of this issue - give yourself the freedom of having a deadline, and cutting out or defering work that prevents you from meeting that deadline.
And FYI, having gone through this many times at work - the best time to start project management is at the start of the project. The second best time is right now - don't wait for some milestone. Just organize all your outstanding tasks into a backlog, divide up your time into equal sprints, plan the remaining work, and set a deadline, then don't add any more work until that deadline is met. If you must add work, remove other work. The deadline is your friend, it's where your goal is, it's what organizes your work and lets you decide what's important.