Wow. Here we are. The grand finale of an amazingly beautiful and complex series. I could cry. What a journey....
This is a really, really complex plot. I mean, whenever you're dealing with time travel, it's going to get complex. This is no exception. Typically, time travelling plots always break down because you're always creating a paradox. Avengers: Endgame tried to rectify this, and I'm not they completely succeeded, but, before them, CLAMP attempted the same thing, so the concepts in both are similar. If you've seen Endgame, recall what the Ancient One says to Banner about timelines, because that's exactly what CLAMP is discussing in this book, only they take it one step further and decide to boldly do what no time-traveling series has done before: own up to the paradoxes they created and face them. That. Gets. Crazy.
This volume is a roller coaster time-wise. Mark which Sakura/Shoaran are doing what and why. Try to follow the purposes and goals of Fei Wang, Yuko, and Clow Reed. If you can, reconstruct the entire overarching plot in order so that you understand what happened and how it affected time. Keep in mind that when you're outside of time, like Fei Wang and Yuko have been up to this point, you can see the entire thing and influence various points of multiple strands of time. Fei and Yuko jump between timelines often to achieve their (opposing) goals. And don't forget Watanuki either!
I highly suggest, once you finish this series (or once you finish xxxHolic which ends after Tsubasa ends if you're reading both together) that you immediately pick it up and reread it, marking from the beginning which Shoaran/Sakura is there and the objectives of Fei Wang and Yuko (as well as Fai D. Flourite's knowledge of events). I think it'll help the reader understand exactly what's going on. Chart time if you can. And enjoy the fact that you've just finished one of the most complex, detailed, and beautiful manga that exists. It's confusing, especially towards the end, but keep at it. Reread things. I was totally lost when I read this years ago, but, coming back to it, I think I get it better now. Still some questions, but I think a reread will help.
And, let's not forget, this is just the beginning. A sequel series is partially complete with hints of intersecting with the new Cardcaptors as well. So....the journey continues. I was not at all disappointed. My memory served me well. Still one of the best stories I've ever read with brilliant characters, deeply complex plot, gorgeous artwork, and all sorts of feels. Well done CLAMP. Now please, PLEASE finish x/1999. Pleeeeease?????