There are prequels done by Brian Herbert, Frank Herbert's Son, and Kevin J. Anderson. Some fans like them, a lot don't... My opinion, none of them were as good as the original books. Yet, I found of the ones I did read they weren't too bad and they filled in some stuff.

I don't think any of them really touch much on the terraforming of the desert. Maybe some in the House Trilogy with Pardot Kynes. I can't remember.

As for after the third book it being weird, I can't remember where I read this or if it's true at all, but I believe I read that the original three books were to be considered one story. Then the books after that another.

The prequels are setup different and don't exactly take place on Dune itself.

The Butlerian Jihad, Machine Crusade and Battle for Corrin were one set. They focused on the distant past and setup how the world would come to be. Showing the war with the machines and setting up for the Bene Gesserit and Mentats and how the main families of Dune came to be.

Then in the House Trilogy each book focuses on a House. House Harkonnen, Atreides and Corrino. These were written as the original prequel series. These are focused as an actual prequel in which the main players are meeting and the backstory of some of the favorites like Gurney and Idaho...

You also have the sequels where they finished the story. Hunters of Dune and Sandworms of Dune.

All of these are supposedly based on the notes that Frank Herbert left behind when he died.

There are also in between books such as Paul of Dune and Jessica of Dune. These books fall into the original storyline but try to fill in gaps between the books. I've never read these though.

You also have another series about the other groups. Sisterhood of Dune is the early years of the Bene Gesserit. Mentats of Dune is the early years of the Mentats.