This is my gotcha: how do you decide where a document go? The categories might be clearly split, but documents typically won't fall in exactly one category.
How do you manage temporary tags? That is: how do you operate on the documents? You might be having some documents for your "2020 tax return" in several categories of your system. And what now? Do you duplicate them in another category (tax return?)? Do you tag them? How do you keep track that you moved them? (In case they're real objects, like pieces of paper).