Sub-categories with Discourse

I’m not really sure if its belongs into this category or to the theme category so I hope I don’t pass any rules here. Also, I sorry if my question is not so understandable. I feel a bit confused with the design part of Discourse.
I see that Discourse is designed to have main categories that are actually tags. I was wondering what if someone wants to have a forum with sub-categories.

For example, consider a forum which deals with Math courses. In the “academy” category we have sub-categories like “set theory”, “calculus”, “algebra” and so on. Other category “school” have subcourses like “geometry”, “basic algebra” and so on. How discourse can handle it? It feels like it gets a bit messy.

Also, I’m not sure if it’s really the right way to create such forum. Maybe it really should have “main categories” like “school” and “academy” and tags for each one of them.

Would love to see a suggestion for a theme/plugin which handles with it or hear your opinion about it.

This might help: It's Time We Talked About Tags