I think this raid going along so smoothly is a red herring for something unexpected happening that really makes it seem like all hopes lost. Imo I think it'll be a bit similar to what happened in Thriller Bark, like how easily the SHs took out the zombie generals to later fighting Moria and Oars. I think all the beast pirates will be taken care of and Kaido and Big Mom will be the only ones remaining.
Now with how they'll deal with them, idc about how Big Mom's handled but I think it'll play out like this:
1) Luffy puts Kaido on the brink but Kaido comes out on top and then the SHs hold back Kaido and Kaido beats them all until finally Luffy does what he says he'll do or...
2) Luffy thinks he beats Kaido and Kaido does what he did to Oden and blindsides Luffy and it looks like all hopes lost. Luffy will be out just like he was out in Thriller Bark after the Kuma bomb and this is where ZKK comes in.