Mark, Everything you say makes sense. I hadn’t thought of a tag cloud, and was trying to figure out how to simulate a tree using tags. But of course, you are right, a tag cloud is better.
FYI, in my old forum, I have no actual idea how deep any of the tree branches would have been, since they depended entirely on what a user might do when posting a new topic or answering an old one by splitting it. I suspect that many would have been only a couple of levels deep. In Discourse, I gather that one can reconfigure the category/sub-category structure without losing existing messages, but I’m a long way away from doing that.