Actually only 2 books are follow ups, 3 are in between of Dune and Messiah and rest are prequels, plus iirc one book of stories, that one about Gurney was quite fun. One book per year is nothing out of ordinary nowadays, that's how the publishing houses crunch authors, hell even for example Terry Pratchett was churning 1-4 books per year for most of his career. Also idk how they are dividing the work, but i would imagine most of actual writing is done by Kevin Anderson since he was a real deal crunch writer even before starting cooperation with Brian, there's two of them.
Oh and also look for the 3 volumes graphical novel of first Dune book by Raul Allen and Patricia Martin. It was directed by Brian and it is seriously the most faithful adaptation of the book ever made.