I have re-read it in my late 30s and now I actually favor the later novels. I consider all 6 novels as one work.
The deconstruction of a hero (Paul) was in my opinon necessary and logical progression of the story arc. I think Herbert wanted to say that heroes inevitably end up being harmful to the society.
My opinion is that if the sequels were written in the same manner as the first book, we'd end up with an excellent but still run-of-the mill epic saga like Star Wars, LoTR etc. In my view, Herbert's Dune stands above all precisely because it's not just another (great) SF novel.
Herbert said he set out to write a book on philosophy and religion, but didn't want to approach the subject academically since it would probably be only read by scholars. He wanted to reach a broader audience and so he wrapped his ideas into science fiction so it's more easy to swallow for the average bookworm.
Dune isn't a "geek power fantasy" Isekai style, it's a cautionary tale against blind faith in leaders and religion. It's made clear from the start that the protagonist, Paul, isn't a hero, as him and his mother manipulate the fremens to gain their trust.
Unfortunately the original books of Frank Herbert are really hard to convert into movie / TV format and that's why Dune never became as popular as star wars. Part of the reason is the inner narrative and attention to details like facial expressions. Actually what makes the books great is what's stopping them from becoming mainstream.
While their writing is not as brilliant as Frank Herbert's, the books they wrote, mainly the prequels, have helped me understand just how big the original vision was and have helped me enjoy the sequels more. God Emperor is truly as masterpiece, but it took a couple of reads, and the sequels, for me to appreciate it more.
The prequels are basically Star Wars books set in the Dune universe.
The same way KJA "improved" Star Wars.
His wife, incidentally the editor-in-chief of the publishing imprint that then owned the publishing rights to the Star Wars and Dune literary universes, is to blame for unleashing his word vomit on mankind.
(I personally do believe that some notes were left, but I suspect they were not complete, and were utterly ignored by Brian & Kevin. Because they is no explanation which makes more sense. There's no way he would have tied up with series by referencing characters never-before introduced from before the previous books.)
So, maybe? I lean towards thinking there were notes myself, as I can't see anyone making up that ending de-novo.
It seemed to me that the two figures in Duncans visions were going to become important, but they'd be rogue/evolved/advanced face-dancers rather than representatives of the machine intelligence.
I guess we'll never know for sure unless the notes are published.
Given the lengths that Frank Herbert went to avoid having computers in Dune, it would not make sense for his big bad to be a computer revealed in the last few pages of the series, especially when his big bad was always intended to be Destiny.
The author admits to having given up after the fourth book, God Emporer of Dune, which is, to be fair, a fairly radical shift from the original trilogy. I can see it putting off someone deeply invested in the original.