The books are kind of like duologies that each follow a character.
Dune Messiah would be a perfectly good stopping point. I feel like if you go on to Children of Dune, then you kind of want to do God Emperor too because of how Children of Dune ends. The books are kind of like duologies that each follow a character. Dune/Dune Messiah is Paul's story. Children of Dune/God Emperor of Dune is Leto II's story. Heretics of Dune/Chapterhouse: Dune is Darwi Odrade's story. (Wasn't sure how much of a spoiler the main characters of subsequent books would be considered, so spoiler tagged them just in case)