Dune does go downhill after the first book, unfortunately. You don't sound like a snob. You sound like you don't want to waste your time with something that would not interest you. That's called knowing your own mind.
One small warning about The Gap Cycle. Unlike Dune, the series gets better with each installment. The first book entitled The Real Story is really a mini-novel. Some people are turned off to the series with this first book. It's rather graphic and dark, and might seem pointless, but it is important for the rest of the series, as you will find out later. My advice would be to at least read the second book, Forbidden Knowledge. If you don't like that one, you probably won't enjoy the rest of the series.
Good luck in your search for other ground-breaking sci-fi, and I hope you find The Gap Cycle to be a compelling read, as I have.