One of the great things about Dune is the way it works well as a standalone story with a satisfactory ending that, while leaving room open for more, gives a sense of completion to the story.
When he wrote it, did Frank Herbert plan for Dune to be a standalone story, or was it always meant to be part of a wider series with notes and chapters meant for Dune being saved for a future book and a clear multi book arc defined for his main characters in the same way as Robert Jordan, JK Rowling or Brandon Sanderson did/do?