Hello! I'm excited to see progress on your project!
In addition to draw.io that's suggested in the previous post, I have a suggestion for another tool to aid in laying out your story. It's called Twine (twinery.org) and it's great because it is flowchart-y too (although not as fancy as draw.io) and you can test run your choices here since it is a tool for writing interactive stories. It's also free!
Great job so far and looking forward to more!