Yeah, it's not possible to add bookmarks to a complex folder structure on Android. You can resort to saving all to the mobile bookmarks and process that later on computer and / or set a few inboxes as speed dials and have quick access to them via the speed dial tab.
The funny NOT FUNNY thing is, that searching bookmarks on Android works better than on desktop, as:
it is possible to open and navigate into folders from search results
the folders show the number of bookmarks that are contained
The problem with bookmarks panel on Android is, that even though it has the PLUS button for adding a bookmark, suggesting it would be possible to navigate into a folder and add the previously active tab url there via the PLUS button, this doesn't work, as the PLUS button only adds bookmarks by typing the name and url manually! Who the hell does that these days? Somone's grandma I guess 😉