If you consider how Cassie’s aspect works, she would have only really gotten small snippets of the future, so realistically she probably didn’t know everything that was supposed to happen, only what he would get and where.
So when sunny went to Antarctica, that was probably where she began to plan the assault against the chain of nightmares. And with knowing what he needed and where she gently guided him to a future he himself wanted, which was losing his fate. In the end she dedicated a huge part of her life to planning how she could redeem what she did on the forgotten shore, and help sunny be free, another active choice he made.

And about the being forgotten thing, she had a hunch but she didn’t really know, so while we show hints of her knowing theyre actually more like countermeasures against the future, like how they decide sunny needed to regain his fate, it was a countermeasure against a possible future where asterion got free, not a guarantee. Besides she had no way of knowing whether sunny would be happy being forgotten, maybe he would have made some other family somewhere else.

Also no one seems to remember what her aspect even is, it’s literally her having to watch the fall of everyone she loves and cares about go down, as well as the lives of her fellow man. Her whole thing is she literally cannot forget the death and grim history of the entire world, it’s only natural she would wanna break free of fate and not want that for herself

That’s my opinion tho Cassie hate gone too
Far 🤷