The first book has by far the most conventional plot. In many ways it has the shape of a heroic journey which is conventionally digestible alongside all of the quirks of the Dune universe.

The follow up books discuss why putting all of your hopes into a heroic leader is a terrible idea. Paul is considered a failure by the books, and his son’s journey to being a leader and what exactly he means to accomplish gets weird. (In a dense and to many, highly enjoyable way).

The original series eventually delves more deeply into why the society and technology is in the state we see it during the books, but it's a gradual shift. Character focus remains the main theme throughout.

You might enjoy some of the "prequel" books written by his son, Brian, and Kevin J. Anderson (allegedly based... perhaps loosely... on Frank's unpublished notes). They tend to focus more on the setting as a whole. Personally I'm not a fan of the actual writing, but there's a ton of lore crammed into them.

Personally, I feel like the books deal more with how people interact and behave in a future environment. Maybe Brian(the author's son)'s prequels deal with that better, but some people hate them.