Scapple is what you are looking for. Its from the same crew that made Scrivener. You can use it free for a month I think or pay like 20 bucks for a lifetime license. It lets you start anywhere and is very simple and intuitive. The only thing bad about it is exporting data out of it.
https://www.literatureandlatte.com/scapple/overview
This is sort of the problem I have with most things that aren't just a text editor. They may be better in some ways (and I do like Scrivener) but at the end of the day, when I'm done, I want everything to be in a universal format on my own filesystem.
