The third Dune book (arguably the second) actually illustrates this general point about decline as well. Run screaming from the subsequent books

No, not run scremaing! God Emperor and beyond are awesome. But they're not the same thing, no. They're far more philosophical, and kind of about something else.

The Brian Herbert books are very bad.

I think the second and the third was okay, but the fourth was... hmm maybe "decline" is not the right word, it was as if I walked into my favorite coffee shop and they decided to drop a bunch of bricks and PVC pipes on my laps because that's what they do now. (Can't talk about later books - I kinda stopped there and never picked up again. Maybe I should.)

God Emperor is a slog, but sets the groundwork for Heretics and Chapterhouse, and they're in my opinion very good. I've re-read Chapterhouse more than any of the others.

But I think he meant for there to be a final one to follow it, and so it closes off somewhat unsatisfactorily.

I always tell people to read the first one and stop or grit your teeth and read all of them…

(Possible spoiler:)

I was a little surprised in my last read through how small a part Duncan Idaho played until I got to the second book and remembered