Let's try to break this down.
If the "Harley Texts" have been around for "generations" of Giants, I think that most likely means that the "Harley Texts" actually predate the Void Century. Jarul has been alive for 408 years. So, if something is old enough to be shrouded in mystery to even his generation or his grandparent's generation, it's most probably well over 800 years old. Probably dating back to "The Age of Heaven", over 1500 years ago, at least. A time when even the World Standard Language wasn't in effect, yet.
https://onepiece.fandom.com/wiki/World_Timeline#Calendars
Which, I think, most likely means that the World has been destroyed twice even BEFORE the Void Century.
If we expand on this idea, that could potentially mean that Joyboy of the Void Century was actually a "failed" Nika. The World as we know it is in the state it's in because Joyboy failed to destroy and recreate it, as it was supposed to. It's possible that the World was supposed to be destroyed and be reborn, but was actually interrupted in that process by the forces that would become the World Government and possibly Imu themselves.
It's possible that Joyboy is the Nika who "broke" the cycle of planetary reincarnation somehow, causing the problems we're seeing now.
And we really have no way of knowing if these are the only times that the World has been destroyed and reborn. The "Harley Texts" mention that it happened twice. But, it might have happened a lot more times before that, and it just wasn't recorded in this specific document. And there are hints throughout the story that there have been multiple incarnations of Joyboy/Nika.
And if we assume that every silhouette we've seen of Nika is actually a different incarnation, we actually already have a few already.
There's the new "Sword and Shield" Nika we just saw in Chapter 1136. This one seems tied to the Giants, and possibly the original Oars, since they did mention Oars this Chapter.
There's the "Sword and Spear" Nika from Chapter 1018. This one might be tied to Wano.
There's the "Bare Knuckle" Nika from Chapter 1115. But, if you look closely, this one may in fact have some sort of gloves on, it's hard to tell. But, gloves would kind of make sense, since Luffy seems to have a attraction to Boxing Gloves during the Davy Back Fight. This is most likely Joyboy from the Void Century, given the context of Vegapunk's message.
There's the "Skypiean" Nika, who's the inspiration for worshipping Kashigami the giant snake from Chapter 287. This may have been started with another one of the Nikas we know. But, it's worth noting that we DO see a silhouette of someone who looks a lot like Luffy but has wings using a Reject Dial in a flashback scene in the anime in Episode 169. This is technically a non-canon "filler" scene. But, it may inform where Oda is going with this. There may have been a "Skypiean" Nika.
There's the "Bible" Nika, worshipped in the Church that Kuma and Bonney live in around Chapter 1098. One of the other Nika reincarnations may have started this church, or it could have been started by someone who wasn't a Nika reincarnation. But, it's also possible that the worship of Nika was inspired by another incarnation of Nika, other than these other 3.
And, though it's not confirmed, something managed to wake up Emeth the Ancient Giant Robot and caused it to attack Mariejois 200 years ago, as shown in Chapter 1067. We saw that Emeth could be powered by the Drums of Liberation. This indicates that there was likely another, unseen "Secret" Nika 200 years ago.
So, that's potentially 4 Nikas, at least. And I might have missed a few silhouettes.
So, I think it's very possible there's even more "Nika"s that we don't know about yet. Nika may be reincarnated every few hundred years or so. In order to destroy and remake the world anew. But, that hasn't been allowed to happen for the last 800 years, and it's been stopped every time. Causing problems and "breaking" the way the World is supposed to function.
So, I think it's possible that there's been dozens of Nika reincarnations over the millennia. It's hard to say if that's how it works. We don't have a lot of information yet. But, it does feel like that could end up being the case.
And if it IS the case....this may mean that both versions of Romance Dawn are actually canon. The versions of Luffy from the prototype versions of One Piece that we know about, may have actually existed in the past of this story. The one who met Silk and the one who met Ann may both be canon to the main story. I mean, Oda already made "Monsters" canon to One Piece. Why not every other manga he's ever written?
