A good "outliner" can take you very far. You can start with a good, logical outline for what you want to write, then just fill in what you need. There are also

A good "outliner" can take you very far. You can start with a good, logical outline for what you want to write, then just fill in what you need. There are also tools like https://gingkoapp.com https://gingkoapp.com which helps with structured writing and allow to move or "reattach" entire subtrees of a document. My personal favourite is to use lots of separate text files (.md or .tex) and a top level "main file" which includes the desired sections. I find a single level of includes works best since you can see exactly what is going on in the main file, whereas if you include files that include other files, things get complicated real quick! Pro tip for working with multiple .tex files: if you start each of the included .tex files with %!TEX root = mainfile.tex the multi-file authoring experience becomes much better ;)