Kidd and Law getting a flat 2 billion each if they take down BM is within the realm of possibility. As is Zoro getting over 2 billion after ZKK.
This.
I think Luffy finishes Wano with 3-3.5 billion bounty even after ZKK, while Zoro got 2+ billion.
Not exactly.
You need to remember that the Marines treated defeating someone 1-on-1 differently than killing off someone.
Look at Luffy vs Crocodile or Luffy vs Lucci, for example. Both were 1-on-1 fights where nobody died, but Luffy got a sizeable boost after both victories.
While Teach, who (in)famously killed one of his own bosses (Thatch), who was a YC4, still got 0 bounty from the Marines.
Think about it.
Luffy got an additional 200 million from defeating Lucci, but Teach got 0 for killing off a YC? Why is that?
That's because defeating and killing off someone are different for the Marines. Luffy had to surpass Crocodile and Lucci to beat them, while Teach only had to kill Thatch.
Luffy has to surpass Kaido, however momentarily, to defeat him. Zoro does not. Zoro simply has to kill him. Luffy defeating Kaido 1-on-1 is not worth less than ZKK.