Have you looked at
GURPS Thaumatology: Sorcery? The entire premise of the work is building a magic system using advantages. If you're going that route, I'd suggest also picking up most of the
GURPS Power-Ups series, especially
Power-Ups 4: Enhancements and
Power-Ups 8: Limitations, but all of them can be helpful for flavoring advantage-based magic.