Why not use tags? Tags can be created by anyone, and clicking on a tag effectively takes you to a sub-forum made up of that tag’s posts. It also allows you to set up notifications specific to that tag and create new topics with that tag already appended. You can see an example here with our “fantasy” tag.
Speaking from personal experience, when you have hundreds of categories, performance is affected. The discourse team has done a great job of mitigating since our community overwhelmed the system, but it is a concern to consider.