This could've been written better. The angst feels off because I don't feel like Setsuna would ever act like this. If I can't buy the characters in that situation than I can't invest into the story.
Ikr, I feel like Ayumu and Setsuna swapped personalities??
I think maybe it would have been better if Ayumu was the one who see the red string and her soulmate it's Yuu, but Yuu loves Setsuna
I must disagree. This is very much in character for Setsuna. She makes a decision she believes is best and pursues it with bullheaded stubbornness. And the more selfless the act, the more stubborn she is, driven by her desire to be a hero like the ones she obsessed over in anime.
One selfless canon example would be disbanding the first idol club. She did not consult the other members. She decided for herself what she believed was best for them all and stuck with it until confronted about it.
A more selfish example would be defying her parents to be an idol and watch anime. She has made her decision and pursues it regardless of their desires. (SIFAS may have included a bit where she mentions having pleaded her case at least once, but I cannot recall with certainty, and the anime glosses over the issue, so unless that changes in the second season, we are free to assume either way.)
Here, Ayumu unfortunately cannot confront Setsuna over her decision as she cannot see the fate thread and is blissfully unaware of the decision being made on her behalf.
Might it be better if the roles were swapped? Perhaps, perhaps not. Either way, this author chose this arrangement and I can see it working for these characters as portrayed.
Am I biased toward this view? Absolutely. My own fic has chapters that, though unwritten, are still fully planned and outlined, that see Setsuna making similar selfless decisions, or at least perceived by her as selfless, and executing them without a word to others who will be impacted by her actions.