I don’t see how he isn’t Luffys goal? Roger is the only PK we know of and Luffy wants to be PK. Zoro’s goal is to because the WSS he only needs to surpass Mihawk to do that Roger is beyond that. Mihawk himself said becoming PK is a more lofty goal than surpassing himself. Then Luffy stated he needed to be able to beat the admirals and Yonko to be PK. Your going against the story.
Boy oh boy, I need to start charging money for the services of explaining manga to people...
First of, as far as I know Roger never wanted to become Pirate King, he ended up being called like that while Luffy wants to be the Pirate King. Roger just found the treasure while Luffy will claim the treasure. Roger was avoiding the Emperors, Luffy isnt. PK isnt even Luffy's ultimate dream, it's a side achievement. There is plenty different about Roger and Luffy if you even focus on pirating achievements.
When it comes to combat strength, Luffy can't be compared with a swordsman, he can compare to his grandpa Garp who has the same combat style. Roger is a swordsman and Oda wont make any swordsman stronger than Zoro, he wont make anyone stronger than Zoro...
Surpassing Mihawk is surpassing Roger as well. Roger has nothing over Mihawk while Mihawk has things over Roger...
Becoming PK is harder than becoming WSS indeed and that has nothing to do with individual strength. Just the fact that reading poneglyphs is a barrier that nobody can overcome, except Luffy(thanks to Robin), is what makes PK harder than WSS. Zoro will beat Admirals too and kill a Yonko and beat a guy stronger than the Yonko Shanks... I am very much on point with the story.