How can I extract bookmarks, and their sub-bookmarks, as individual PDFs?

I have a large (9700pg) PDF that has 15 top-level bookmarks as well as hundreds of sub-bookmarks within those 15. My goal is to have each individual bookmark as a separate PDF.

I tried split document under Tools>Pages, but that only allows me to split into the 15 top-level bookmarks. When I open each of those 15 new files all of the children bookmarks are gone. Any ideas?