the whole story behind steve and nancy getting back together is for nancy to work on her trauma when it comes to barb, including steve, to realize and come to terms with the fact that it was neither of their faults, because when it comes down to it that is the crux of everything. vecna feeds on guilt and trauma, yes music helps bring them out of the curse when it becomes an emergency but if max, nancy, fred, and even jason's friend, were able to work through their trauma vecna wouldn't even care to go after them to begin with. we see it with max too. she is severely depressed, she passes everything off as "i'm fine", she doesn't tell anyone what's really going on, not even the school therapist. but it's her finally breaking through, finally realizing that she's not at fault for what happened to billy, that wanting harm to come to billy because of all the harm he brought her wasn't some self fulfulled prophecy, that she does have people, family and friends who love her and want her in their lives that is what the music triggers in the first place. that is what nancy has to work on with steve, which she can't do if she's with jonathan. he was her escape, he was her running from the compicated guilt and trauma and pain that came from continuing to be with steve, but she has to face it at some point.