I don't think it matters which 'elements' of Star Wars weren't original. Lucas was able to synthesize and use them in a completely original way. That's what storytelling boils down to, really.
The thing is, some elements are more exact than a "dethroned prince struggles against massive odds to conquer his enemies and reclaim his throne" story - which is a large part of what Dune is, at least the basic story.
It took *less than a year* to conquer the galaxy? From his visions I assumed it would take decades or centuries. That's just completely immersion-breaking.
Where do you get the "less than a year" from?
There are TWELVE years between the two novels. The conquest is still ongoing at the start of Dune Messiah, although by that time it's just mopping up straggler-worlds and pockets of rebellion. Of course rebellions are ongoing anyway; that's how it is in most empires.
The Imperium is not the entire galaxy. That said, I myself raised these doubts on a couple of Dune forums that it seemed impossible to conquer the Imperium and remake Arrakeen in just a dozen years. But considering that they had foldspace technology (FTL, for all practical purposes), a vast number of Fremen soldiers, the threat of atomics (the Atreides were willing to defy the Great Convention if it meant getting what they wanted), and a prescient leader who could see the future... not to mention the new religion being developed and disseminated by Alia and the Qizarate (a new class of Fremen priests) into an Imperium that had gone stale and jaded and corrupt, and that was all it took to kick this into high gear. Consider the vast undertaking of building the Pyramids, and that was with without modern tools and techniques. Add in tools and techniques more advanced than we have now (albeit non-computerized; remember that there are no computers or AI anywhere in the Imperium at this time, since they were considered anathema), with a fantastically dedicated group of people doing these projects, and it could happen.
Oh, and there was that little matter of "he who controls the spice controls the universe" and "he who can destroy a thing controls that thing". The planetary rulers were dependent on spice, the Bene Gesserit were dependent on spice, without spice the Guild wouldn't be able to function and each planet would have been isolated, and many wealthy people were taking the spice to prolong their lives (as well as using it as currency). So Paul pretty much had everyone at his mercy at that point. The various planets and major Schools could either submit or be destroyed.
My great disappointment with Dune was that the first half of the book promised far more than it delivered. I thought that the news of Paul's son being killed and his sister taken would be the turning point; the moment where his perfect messianic story comes crashing down and he would decide that, yes, he would like to conquer the universe after all. How cool would it be if the reason that he couldn't see a way to stop the jihad was because he would snap and become the villain who leads it? But no, here's some plot-convenience for ya: the Baron dies like a *****, Alia escapes, Fremen win the day, and some vague handwaving about humanity going stale to justify the conquest.
The things you're mentioning here are addressed in Dune Messiah and Children of Dune. The truth is that Paul realized that things had gotten to the point where even if he were killed or stepped down voluntarily, the Fremen would continue the jihad in his name. They had been put down and abused for thousands of years by various Imperial rulers (when they were known as the Zensunni Wanderers), and the last few decades on Arrakis under the Harkonnens had been particularly brutal. The Fremen had their freedom, a new leader with unimaginable powers of mind and body, thanks to his prescience and Bene Gesserit training, and the means to impose their will on the people they saw as their enemies.
One of the lessons stressed throughout the Original Dune series is "beware the charismatic leader" - because even with the best of intentions, power corrupts. We see this in our world around us now.
BTW, there are two other books I'd recommend that were not written by Frank Herbert.
The Dune Encyclopedia, by Dr. Willis McNelly, was sanctioned and approved by Frank Herbert (although Herbert reserved the right to contradict anything in it in future novels). The Encyclopedia covers the first four novels, up to the end of
God Emperor of Dune. I can't imagine what McNelly would have made of the last two novels; there are some elements there that rival Heinlein for things we consider immoral in the real-world 21st century.
The other book I recommend is
National Lampoon's DOON. It's a parody of Dune, written in Frank Herbert's style, and it's hilarious.