What if Sona worked like an actual musician? Beats instead of cords.
Simple idea since i have limited time to type. All her abilities constantly refresh "beat" with a set cost per beat, on top of a flat cost to change between them (you may recast the same beat to stop playing). You can increase her base cooldowns and remove the passive bonus from her ultimate, thus removing her ability to chain gun spells together... thus also removing her ability to set up her passive 2-3 times every 10 seconds. This would require Sona to manage her mana since leaving any of her abilties running would rather quickly drain her pool, but it also allows for Sona to be played more passively since her focus would be more on buffing rather than poking early and chain gunning spells late game. If the mana costs turn out to be to low, you simply put in a system where the mana costs increases per ally effected (up to a cap?).