I understand it can be hard to restrain ourselves from over-engineering, but keep in mind that Discourse is very flexible, and that you can tweak your categories afterwhile. Which includes merging categories, moving posts from one category to another, replacing a category with a tag and vice versa, etc.
So, it’s not a major issue if you think you made mistakes, or if you think your categories and tags organizations could be improved after launching your forum:
Of course, it’s always ideal if everything’s right off the launch, of course. What I’m trying to say is that these tools can ease your overthinking, since what you come up with can always be changed afterwards.