My method is 100% manual. I have a set of tags used, pretty general ones: crime, thriller, romance etc., less than 20 in total. I do my best to use only those tags, unless I think adding a new tag is really necessary. From time to time, I may add some subgenres/subtopics, e.g. fantasy.witches, fantasy.dystopia etc., depending what kind of books I feel like I should separate into subcategories. If I'm not sure what the genre of a given book is, I find some information online, make a decision regarding the genre
and enter it manually. I don't download tags but sometimes I do end up with lots of them if ebook files are tagged... then I leave what I find right and delete the rest. Oh, and in fact, I don't use the tags column for that but a custom column #genre.
For non-fiction, I have a whole custom column, with hierarchical categories of topics/areas (e.g. science.chemistry, science.biology, humanities.philosophy, linguistics.phonetics etc.).
For me, it works just fine, especially as I use those genres/categories more frequently to browse through an entire genre/category than to check what genre a given book represents. Therefore, the general approach fits my needs better than lots of very detailed tags.