If we were to completely follow the lore and take it as truth then Calus has no special sneaky ulterior motive at all, he was exiled on the Leviathan which was a prison ship at the time, not long after being a prisoner the ship ran into an anomaly/darkness that killed anyone that went outside to investigate, Calus decided to go full big dick energy and casually went outside regardless, everyone thought he was dead until he rocked up hours later claiming he'd seen the end of everything in the void.

This vision/experience instantly changed him into Big Emperor Party Time, he realised the futility of any and all actions between now and the end so went all-in on everyone living their last days in excess and opulence. His only apparent goal seems just to try to be the last alive to witness the end, not to be the only survivor or the winner, he knows that's not possible, his testing and favouritism towards us isn't some grand background plan, he just wants to find the most powerful being who can give him some entertainment before the end and who he deems worthy of standing beside him to witness that end, someone that gets to be the 2nd to last person alive in the universe (yay!).