Messiah was actually part of the first dune in the original design, although Herbert later splitted it.
Children is something else, even if it shares the characters, it’s very much a setup for God Emperor. It has very different themes and style. Same with Heretics.
IMHO Dune is better seen as three duologies: Dune+Messiah, Children+God Emperor and Heretics and Chapterhouse.
I love Sufjan Stevens, but I don’t see the comparison. While I really love his lyrics (one of the few I actually like them, I usually find most artist lyrics to be plain and way too cheesy), his music is very simple compared to a behemoth like Mozart.
Mozart was able to write highly complex music very fast, that went from deep themes to silly ones, and enjoyed popularity from both critics and public, which is something quite rare.
I don’t which one would be the closest today. Maybe something like Williams or Ennio Morricone.