- mark a forum, or a sub-forum, in order to follow the subjects on their entry page

Forums and sub-forums are the structure of your website, a regular user should not have access to manage them, this is out of all forum rules. They just need to subscribe to forums in order to follow. They can find the button in each forum page next to the title. Also, they can subscribe to any forum in their Profile > Subscriptions Tab.

- do the same with the subjects, whether they are theirs or not;

Saying subjects I assume you mean Topics. And yes, they can follow to topics by subscribing as well. Here is the documentation: https://wpforo.com/docs/root/topics-posts/forum-topic-subsciption/