The first three books are Paul's story, and that of his children's rise to power. Those are pretty self contained. Book 4 is the end of the Golden Path, where Leto II is God Tyrant, and rebuilds humanity to have freedom from superhumans like the Kwizatz Haderach. This is a sort of big sequel/addendum to the first three books, and all four tie up Paul's story, and Leto II's story that is started in Book 3 and finished in book 4.
The series starts to drag after that as we go further into the future of humanity, and see all sort of things getting progressively more convoluted and bonkers, and I'd only recommend those to the completionist. They are not terrible, but don't reach the heights of the first three or four books. Book 5 and onwards certainly feels like Herbert is riding a successful book series, and doesn't want to get off until the fans stop buying, and it sort flatlines until you get to the Brian Herbert/Kevin Anderson novels which IIRC take a notable dip.
I'd certainly recommend the first four books. After that, if you've had enough of the Dune universe, I'd stop there, or if you really want to delve into the deeper future of the Dune universe on the chance it's hooked you in, then keep going until you get fed up. If you start to see a drop off in interest, it's not likely to pick up for you as you continue down the series.
I don't think I'll be reading the books, just not my thing nor do I have the time. Is there a time jump in any book 1 since the movie ended about half way? Also do you think part 2 will cover only upto end of book 2?