There's a total of three endings in the Sylvia route:
1) The main route ending when you suggest Sylvia hypnotize herself
2) The alt route ending when you don't suggest Sylvia hypnotize herself
3) The short end when you walk out of her apartment without letting her help you
If you've done those three paths then you've seen all her endings. And as far as fully becoming Peggy, that depends on how you feel about the main route ending.