The Culture are very over-indulgent as they pretty much indulge their every whim. Because that's all they have to do.

Anyways Slaanash does have daemons that literally try and get artists to strive to make a perfect work and feeds off of them when they do try.

So basically any over-indulgence will do. Working to become the perfect swordsman, an obsession with winning a game, even just the constant listening to music. Slaanash draws power from moving past restraint (which is why the Eldar's Paths are entirely about restraint.) no matter what kind it is.