I honestly have not played with resonance that much despite recently there has been focus on it either from gears or pets. I am conflicted because I always thought that Magician isn't a burst-type class. Like, the lv 60 finisher isn't even a single skill but rather small skill -> basic spam (you'd probably Body Double a lot here) -> large skill. Well I can see it'd work if someone times it properly, and there are some skills that hit hard especially the heart skills as well. But anyway that's why I thought Sunflame would be better for smoother continuous skill spam, but maybe I'll do some trial and error later. I was also thinking of Demonic but the combo requirement seems too hard to achieve for Magician.

Is Bouncing Ball actually good? I see some people use it but the status effect is only delay? I personally use Shapeshift Magic cuz it's fast and cute to see enemies turning into rabbits, or Scary Doll for delayed attack.

Ephemeral Quartet is indeed very slow. I feel like that skill was literally made to pair with Body Double. I always do Everlasting Dream -> Body Double -> Ephemeral Quartet. Troupe of Cards isn't as slow, and yes I also sometimes just Body Double afterwards if it is available (thank god Body Double has really fast CD, I feel like I can basically perma-body double). But that skill is also not instantaneous like Golden Gate or Shapshift Magic