While I don't have any particular experience with SuperMemo, I've struggled for a long time with the best way to categorize large amounts of information. A tree hierarchy in general I find is insufficient because there's so many things that have cross-cutting concerns. For example, if you have a topic about Flight, and you have some books and some videos, and you also have a section for Books, can you take an entry and put the single entry into both Books and Flight levels of the tree?
In the past, I've found wikis to be a great way to do this, namely due to the tagging system, because you can easily create those kinds of relationships with tags. Wiki markup has largely fallen out of favor for Markdown instead though, but there's no good markdown style wikis out there (and I've looked, and considered creating my own).
In my tech career I've been working with graph database (Neo4j specifically, based on the Property Graph Model), which is really impressive technology and I think would be a great fit for something like this, but alas I haven't found any off the shelf software to help model and categorize things this way.
