I’ve was toying with how to use joplin to write a book, organizing a notebook where notes would be chapters. I could implement this PDF thing easily, but the problem is the ordering of the notes for that. But for your use-case, I could do it easily if you’re interested. Basically I would create one note for all the notes in a notebook concatenating the notes and saving it as a new note that you could then export.
To solve my use-case where the order matters, I thought about making a note using the copy markdown link feature and then export all referenced notes in order, using the same idea.
It would be cool to be able to export that to epub too.
