"Destiny" as a concept does not really exist. Nor does it contradict a character "struggling to reach his dream".
Just think, why was it even Luffy's dream to become the pirate king?
Well, what's a fact is that the one piece world needs someone who finds the one piece and challenges the world government. This can only be done by the "pirate king"(a title the world government made up)
Luffy is the next Joyboy because he will be the same to the world, play the same role, do tue same for the people as the historical Joyboy.
"Nika" is just a name for now, and we don't know who invented this name. It might have been the world government or even the ancient kingdom.
Luffy will keep calling his attacks "gomu gomu no...." and won't call himself Joyboy either.
He is Luffy and people only call him the next Joyboy like they call Zoro the next Ryuma.
Of course Zoro is not Ryuma. He doesnmt have Ryuma's sword currently yet people call him the second Ryuma.
His role as such is neither a direct result of his bloodline nor is Ryuma's sword imposing its will on Zoro or anything like that.
And all the same, the devil fruit is not all that's necessary to make Luffy Joyboy.
And: Roger was only waiting for someone because he got that info on Laugh tale.
There might not even be a "prophecy" in the traditional sense.
Joyboy probably had voat and possibly future sight. He might have just been capable of looking that far in the future where he saw someone being born 800 years later to fulfill what he couldn't do.