The Problem With em Dune: Part Two /em

I have questions about Denis Villeneuve's Dune: Part Two. If the Fremen have lasers, why don't they just shoot the sand harvesters and run away? Why don't they use their sandworms until the last battle? Wouldn't it make more sense to fight the other great houses on Arrakis itself, where they have sandworms, rather than board ships off-world to go off to war? If Paul (Timothée Chalamet) has to invade the galaxy at the end, why bother marrying the daughter of the emperor he just deposed?