Going to be honest, those examples at the top just feel like they've been pulled out of thin air.
The ending of the first QC epoch has to be "The Talk", it signalled the change in the fundamental relationship between Marten and Faye, as well as the relationship between Marten and Dora, triggering their relationship. It also saw "the bit players" gain more substantial roles and storylines of their own.
The next major landmark and thus the opening of a new book would most probably be "The Breakup", because again, it fundamentally changed the relationship between the characters. It also saw several of the characters begin to mature or at least become more developed (Marten's fling with Padma, his experiment with a one night stand, Dora entering therapy and so on).
The end of the third epoch is marked by Faye's descent in dangerous alcoholism and her near death experience. It changed the core of Faye's character and forced her to take stock of herself and her relationships with everyone. She lost her job at Coffee of Doom, but managed to keep her friendship with Dora. She lost her relationship with Angus, but seems to have found a healthy one with Bubbles.
When there is a fundamental change to a character or to the relationships between characters, it changes the potential storylines for them and thus changes the kind of stories we end up seeing.