He was too weak to beat big Mom in wci alone, he wouldve trained making the arc even longer

Thats not the answer

I dont think it would be THAT HIGH

he would be 2.5 billion after WCI and 3.5-4 billion after wano bc of the five elders wanting to hide the gomu gomu no mi

He got a 1 billion increase for just defeating Katakuri. Kid and Law got 3 billion for defeating Big Mom together, Luffy's would be at least that high after defeating BM and her whole crew.

I think it would have been either 6 or 7.5 billion overall. If he had beat BM it would easily been over 3.5 billion and defeating Kaido later would have been the same if not a bit more. But just how high would depend on the difficulty it took for Luffy to do it. 😎