One thing that’s pretty cool about tags is they don’t all have to serve the same role. I think of them as alternate hierarchies.
For instance, planning a book could be aided with tags for chapter 1, chapter 2, etc. Those tags group Devonthink documents by appearance in the book, not by subject. I would probably put those chapter tags under a parent tag like “book/chapter 1”.
Other tags could group by subject matter.
Yet another tag tree could group by the year an event happened.
For some databases, I enable groups as tags by disabling the negative option, “exclude groups from tagging” in File->Database properties. That is like streamlining the process of creating replicants.