I'm entirely sure he's saved them for last because they are central to his conclusion. They are also some of his most serious characters (plus Mihawk), it was clear Kaido and Big mom were going to be these essentric, weird, grandiose characters that will have crazy shit happen to them.
This is why it's good that they are happening first. Kaido will be done this arc. Big Mom 50/50 or at least it leads her to her end.
Saving Shanks, Blackbeard, Akainu, etc for last is a great thing