Millsap wouldn't settle for a first round exit with the Kings at this point, no big FA will be staying if they get traded there. The organization is trash and they don't have talent, there's no reason to leave the Hawks if that's his mentality.
Unless they form a big three right now and make a push for a higher seed than 8th, I don't think they can build anything around Cousins. Their best case scenario would be landing Hayward and Millsap before the deadline and try to convince them to stay, which would be hard since pretty much every team wants Hayward. I can't see any big FA from this year joining them, no one knows if Cousins is staying either.