I agree with your first part, it was done to ease us into the Yonko fight.
But I disagree about Zoro needing to turn the blade black this arc (it never was specified to happen this arc) or even needing to do more to Kaido then he's done. If Luffy is proclaiming he will win and is sending them back down, then chances are this is finalized and Zoro has another role to play. He did what he intended to do (at least not literally), no need to force him back up there unless Oda wants to