Thanks to everyone for helping me think about this. Here are some more thoughts:
She is very smart and does well with math and every subject. But because things usually come easily to her, she tends to get frustrated when it takes longer to really understand something. So something really challenging would probably frustrate her, but it may be good for her if she gets over that and realizes that she can push through it and figure it out, and it's OK that it's not immediately easy.
She does like puzzling through problems. In Singapore she enjoys word problems the most. The end of Singapore 5B introduced some basic algebraic concepts and she told me that she enjoyed them and is looking forward to more algebra.
I am interested in a curriculum that helps to understand to really understand the "whys" behind math, and not just memorizing the formula or the algorithm. I feel that Singapore has been good about this.
So when I heard about AoPS I thought it seemed interesting.
But as I read about it I'm starting to doubt if it is the right fit for her.
My concern is that, as I said, she isn't passionate about math. Her passion is writing and she enjoys all things language arts. Unless she has a dramatic change it seems likely that she will pursue something related to writing and probably not in math or the sciences.
So realistically, she may not end up doing calculus, and that would be OK. If she wants to focus her time in high school on language then she may not have as much time to spend on math. But I'd like her to understand WELL the math she does know.
I've seen people say that AoPS takes their kid 2+ hours a day. I don't know how common that is, but I could see that being tough for a kid that does well on math but doesn't care that much about it.
But if we don't do AoPS I would still want to make sure she understands the "whys" behind math.
I've also been thinking about my teaching time being more limited with younger siblings coming up behind her. This isn't absolutely required, but it would be nice if whatever we use had more of a "coach" model where she could come to me if she doesn't understand, rather than a teacher model where I explain everything as we currently do in Singapore. That would help me afford more time to spend teaching the younger ones.
Thanks for helping!