Herbert didn't want to continue Dune and was pressured to write a follow up. It was an era when most science fiction was still published in periodicals. The first half of Messiah are the results that were then compiled into the start. It is like a really shitty draft. Everyone experiences the same thing. I put it down for quite a while too. If you can make it to the second half, it will become one you can't put down, like the first. It does setup well for what is to come. After I got back into Messiah, I read all the way to the end of the entire series, even the Brian Herbert/Kevin Anderson stuff. Those last two are not like Frank's writings, but are their own thing and still more readable than the first half of Messiah. IMO the first half of Messiah is a great example of what happens when Art takes a back seat to an anxious banking type mentality. Bankers make terrible artists and advisors.

GEoD is IMO the best book in the series as it eviscerates many cultural norms and deep assumptions like fascist altruism, eternal boredom, the coexistence of misogyny and feminism, manipulation that is both brutal and kind, and if an alien can be human. It even infers the question of potential delusional prescience in my opinion. It will make you think about the motivation of leaders and what you may endure because of their vision of a future.