That's fair. In the meantime, ridiculously enough, I figured out what I "wanted". So, starting from
https://www.goodreads.com/genres/non-fiction I added to my spreadsheet all the "related genres" listed in that page. Same for
https://www.goodreads.com/genres/fiction. And proceeded to use only those on my goodreads.json i.e. Fiction -> Fantasy.
In my mind, that gives me the genres at the 2nd level. I assume that's not fully accurate but is a good starting point for what I was trying to accomplish.
Long story short: from my point of view, Tags are "nice to have", Genres are "needed".