I still have my doubts about her wanting to enter a serious, committed, and official relationship with the MC. Aside from the fact that she just came out of a seemingly long relationship and has been deeply hurt by it, the biggest obstacle right now is that she'll be graduating and the MC has only just started college. Given what she's just been through, she may not want another serious relationship right now since there's a very good chance she'll be moving to somewhere else in the country after graduation and so any potential relationship is not going to last long. Therefore, even if she starts feeling something more, she likely won't act on it in terms of pursuing a more serious relationship and just continue being FWB, at least until near the end where she'll finally own up to it and it will be up to the MC to decide what to do about it.
In some ways, I feel like both Sage and Jill are potentially being set-up as relationships that were foreshadowed by previous ones. With Jill, it would be similar to his parents; a rich girl from a prestigious family gives up the trappings of her family to be with someone of a lower social class. With Sage, it almost feels like it would be a do-over of his relationship with Zoey; there was something of a FWB relationship with them and it was only when she was leaving that he realised he felt more for her but he let her go and didn't let her know how he really felt. So perhaps something similar will play out with Sage where she lets the MC know how she really feels about him and he can choose to let her go like he did with Zoey, or he doesn't and tells her his feelings right as she's about to leave.
