A relatively straightforward task turned out to be a bit more complicated because I didn't find the right page in the official documentation at first. So here's a post to tell you what I did wrong!
When I set up my fantasy blog, I had to configure a Hugo blog at a non-root location on my domain. This was easier than my Flask site, but it still took me some time to figure out what to do.
How to organize your Hugo content folder into subfolders by year without affecting any URLs