Hey, Tobias! Great! Thanks a lot 
I like it that users can freely add tags too. Following your tip, I added templates orienting users to create the tags needed to keep everything findable, acording to each category. I also added some groups of tags to more than one category, and it worked ok.
The tag system is impressive indeed, but when you say it can get a bit confusing, I have to disagree: pretty soon it gets very confusing.
I’ll leave a few unasked 
suggestions in the hope it contributes a bit to you guys:
- for the user, it doesn’t make sense to have three different pages to manage this: one that only serves the purpose of showing the tags; a second for editing groups of tags; and a third for each individual tag. Editing groups of tags and tags (including synonyms) couldn’t all happen within the forum.com/tags page in the admin view?
- there should be a way of categorizing groups of tags: in my case, for example, a category of groups of tags called “Authors” would make ir much easier to find info (these categories could come with their own restrictions, but what I’m asking here is only a better way to visualize this organization)
- there’s no way of deciding the order in which the groups of tags are shown, both on the main tags page and on the editing-groups-of-tags page (they are shown only alphabetically). This way, the first two groups shown for me are “Poetry authors” and “Prose authors”, but the authors’ names start 10 groups below. Ideally, they could all be understood as being within these first two categories.
And once again: thanks! Your answer helped me a great deal. \o/