I think I should read the whole topic before replying. . . .
I think you might misunderstand that categories are collections of posts and that Groups are collections of users that can be used to control who has access to categories.
You can create those categories for each topic of interest. That’s not a problem.
What you can’t do is keep the people who are interested in Music from also seeing topics about Tech.
Oh. Ooops, like this: