Let me be clear.
This guy wrote: "Towards the beginning, it's a series of vignettes that tell individual stories that hold up on their own, followed by huge payoffs that deal with the ensemble cast."
That is 100% not true. It is misinformation. Whether or not there is some sort of bonus when you've finished all of the characters, this is a game about eight entirely different stories, each with a beginning, middle, and end. To say "towards the beginning, it's a series of vignettes" and "followed by huge payoffs that deal with the ensemble cast" is entirely misleading, no matter what may happen post-credits.
By all means feel free to drive yourself crazy hunting for clues that there's some big mystery here, or you can just listen to someone who has seen the credits of the game and is telling you that the characters' stories all end on very conclusive notes, wrapping up their tales and hitting the emotional beats you'd expect from endings. I wouldn't be totally shocked if maybe you unlock some sort of tough dungeon after beating all eight, but each story has an arc of its own. You will enjoy them far, far less if you are just waiting for them all to intersect.
And with that, I'm done posting in Octopath threads until my review is up - see you guys then.