With the system changes to Vivace affecting Magic, Astrology, and Alchemy the Harmonic Saint skill Polyphonic Melody seems a bit silly now.
There's very little advantage to having Enduring Melody, Battlefield Overture, Harvest Song, March Song, and Vivace active at the same time outside of the advantage of having something like BFO + March Song, etc. Making players choose which buff they double up is sorta unnecessary, and preventing teams from having BFO and Vivace at the same time is a mess that doesn't even make sense anymore.
Polyphonic Melody should just be a passive like Chain Cylinder that makes it so when you cast any Music Buff Skill all of the other skills also activate at the Polyphonic Skill Rank's % contribution. This would help the Skill so it shouldn't conflict with Fantastic Chorus' usage anymore and I would even see it go so far as to cast Discord, Lullaby, and Dance of Death at the same time because there's really no reason to give the often mandatory support-type Arcana more to work with.