There's an old(ish) tool by the name of NoteLiner, no longer maintained. If you can find a copy which isn't too buggy (3.3 is the best), I've never found a better outlining tool for methodically breaking down a task or concept. In particular, the use of CTRL+Space as a quick way to collapse/expand a tree is really efficient. NoteLiner is also less restrictive than most other outlining applications, as it's more of a blend of a text editor than something that imposes a rigid structure. Best used in conjunction with George Polya's "How to solve it" method :)

You can achieve a similar workflow in Visual Studio Code with markdown, but it takes a little more work.