When create topic, please force user to choose (0-5) tags.
Each tag comes from different category. Then 1 topic can be displayed in many categories at the same time.
Don’t need to have subforum.
There will be only 1 forum.
And each category will have set of tags. (These are fixed tags which generated by admin.)
And when user create new topic, user will need to choose tags for topic. And the those tags will be factor which categories the topic will be displayed.