We did Frozen Summer Fun last year. Now that the singalong is a standard attraction, really the only thing that's really going to be special is the procession and the fireworks. They're adding a second procession this year, so presumably it will be later in the afternoon, so conceivably you could do everything in a half day. You could easily fit in the procession, the singlong, and the fireworks in the second half of the day. The procession is really short...10 minutes max.
FWIW, you won't need a FP+ for the Great Movie Ride . Most people choose one of the other Tier 1 options over it so waits stay fairly low. We walked right onto it in December.