Then why bring those specific characters?
They’re the characters that are the least relevant/popular so Oda can oeave them out of the arc with the least complaints.
The real question is, why did he leave behind Katakuri and Pudding? Because they are characters that are actually releavnt to the plot and have imprtant relationships with the Strawhats, so Oda couldn’t just leave them languishing in no mans land for the majority of the arc. Instead, the only important character that is brouht along is Perospero, who actually gets to go to Onigashima where he can play a part in the plot.
This works for me only if Big Mom is taken out of the equation somehow for the remainder of the fight, and it just leaves Kaido vs the 5.
Big Mom blatantly has her own game to play here where she’s really after the Road Poneglyph rather than any alliance. Her and Kaido are going at some point to stab each other in the back and weaken Kaido enough to be defeated/bring back OLin, and so all the talk about pirate alliances end in betrayal, the Rocks were all a bunch of backstabbing loonies can come into play.
It just doesn't feel fully setup yet. At least not how Oda traditionally sets things up.
Because it isn’t. That final setup won’t happen until after most of the Beast Pirates are down, like the Birdcage didn’t start moving until everyone bar Doffy was defeated and the Alabasta bomb wasn’t revealed until everyone except Crocodile was defeated. Right now Onigashima is just floating in the sky, non threateningly. We don’t even know if it’s started moving towards the mainland yet.
But when it’s ready for the fight with Kaido to really start, once all the Beast Pirates are defeated, that’s when “beat Kaido before he colony drops Onigashima!” gives us a time-limit for Kaido to be defeated.