The reason Full Circle has the 2-part structure it does is because Kirsten needed to wrap up Golden's arcs before beginning her own.
Full Circle and A Singular Destiny were both parts of the same post-Destiny block of novels, but FC was in development for years before ASD came along. The first half of FC takes place in 2378, beginning about six weeks after Spirit Walk ends, and the second half then jumps forward, skimming over 2379-80 and the events of Destiny in a few chapters, with the back half focusing primarily on the period after Destiny, roughly simultaneous with ASD. In fact, by my estimate, FC ends two days before ASD ends, so none of the book takes place after ASD.