Hi Ned_Gutters, I'm Neorice.
I agree that Trebol seems to be the one who would/should go out last as he's currently sidelining Dofla's fight and doesn't seem to have any clear matchup of his own. (Have we ever had a situation like this in past OP arcs btw? Where the main antagonist had one of their strongest crewmembers actually join in on/sideline the supposed final battle? It feels new to me.)
Between Pica and Diamante there's the question of closing down rebecca/kyros' emotional storyline against a villain that's got obvious human weak points v.s. Zoro shutting down Pika's devil fruit by finding a way to properly damage his less obvious weak spot. (As his real body keeps moving.)
Within the narrative I feel like Pika's the least important executive (Did we even see him in the Corazon flashback..?) aside from his devil fruit remixing the landscape a bit after Clover fainting. So I agree with you that he seems like he'd be first to go simply based on that fact alone.