A plan makes you inflexible. It requires you to exert willpower, which is demotivating and incongruent with the open-ended process of thinking. thinking must produce insight. You cannot force the arrival of insight nor direct it towards a preconceived path. insight cannot be predetermined.
Thinking requires that you adjust your next actions based on new insight, which arrives frequently but not according to schedule. This constant adjustment means that you switch from one action to another when following an insight. It also means that you need to use curiosity to guide your thinking.
Writing that aims at insight must be organized in a flexible system. Instead of a plan, you need a workflow that optimizes freedom (A good workflow optimizes freedom) because freedom motivates us. A workflow that involves a close cooperation with a collection of notes such as zettelkasten and talahardin is far superior than planning.
In a related note, even the Natural planning model outlined by David Allen is not really planning but an open-ended process.
References
Ahrens, S. (2017). How to Take Smart Notes: One Simple Technique to Boost Writing, Learning and Thinking – for Students, Academics and Nonfiction Book Writers. CreateSpace Independent Publishing Platform.
Allen, D. (2015). Getting Things Done: The Art of Stress-Free Productivity (Revised edition). Penguin Books.