One of the core principles of my workflow (inspired by REPL development and some unix tools) is to start with a single file (for a function or the whole project). The I refactor the code to have a better organization and to improve reliability, especially as I'm handling more scenarios (and failure modes).

LLMs are not useful in this workflow, because they are too verbose. Their answers are generic and handle scenarios you don't even support yet. What's useful is good documentation (as in truthful) and the code if it's open.

This approach has worked really well in my career. It gives me KISS and YAGNI for free. And every line of code is purposeful and have a reason to be there.