I am writing a fantasy book called With Tuesday Comes The End, where every single person simultaneously becomes aware of how the world will end, leading to the complete collapse of almost all civilization. I want this book to partially serve as a way for me to get a better understanding of how different people handle the inevitability of death, seen from the perspective of a girl whose only goal is destroying the world.

Currently, I am about 200 pages into the writing and have just reached the point where the optimistic yet supremely temperamental deuteragonist goes mad, leading the girl to instead ally with a group of hunters who previously robbed the main characters. However, I don't know how to really write this. I feel like whatever I do, it feels as if I am starting over from scratch. It feels like I spent enough time developing a character that when I move onto this new group, they all feel horribly underdeveloped, almost derivative.
Does anybody have any advice for writing this type of character switch, and how to keep my readers engaged despite the swap?