Yeah, once you get to the end of the first half, it's like oh crap, we're out of time, montage montage, end battle, fight with Sting, plot plot plot, end credits. The film that's supposed to be released this year is reportedly only the first half of the book; I guess we'll see how that goes.
Standard 3-act structure has a natural breakpoint at the midpoint twist of Act 2, not that my recollection of Dune is detailed enough to confirm that it maps naturally to 3-act structure in detail.
This actually reminded me of a colossal mistake made by Hobbit pt.2 movie, when they ended the part with a dragon flying away to burn the city after the cringiest of chase scenes.
They could have easily end the part with slaying a dragon, and then show it again in part 3. All they needed was to find a different POV character!! One dragon kill, shown twice.
Movie is also great. It's just not finished. :(