Kaido has the same knowledge Doflamingo does. Do you think Doflamingo would be stupid enough to try and hide information from Kaido? Doffy is way too smart for that. Doflamingo knows a giant war is inevitable. It's one of the reasons he sided with Kaido, a notorious war-monger said to be invincible in 1v1 combat, who is prepping for war.
Kaido knows a world war is coming. That's why he chose Wano. Everything Kaido has done is to prep Wano for the coming war, so that Joy Boy has a war fortress. Either Joy Boy showed up in Wano as prophesized and beat Kaido, or Kaido was going to start the damn war himself.
The Gifters were never about power. It was about trying to create an artificial Nika Nika Zoan, IMO. Kaido wanted to destroy the Celestial Dragons the same way Doflamingo did: by destroying their world completely, and turning it into a lawless warzone. Either Joy Boy was going to stop him, or Kaido was going to keep going until he brought the world he despised to ruin.