­Nantuko Disciple says this can be a tiny bit bigger, but it probably also needs to be uncommon these days, as this is quite the onboard trick.

Do you think a clause of say "activate this ability only during your main phase" would make it an acceptable common?

Definately. The whole reason it isn't allowed for common is because it makes combat confusing, and causes many players to forgo attacking in limited. Sorcery speed prevents that from being a problem.

Interestingly, sorcery speed optionally makes it a red ability if you wanted to go for that instead.

Could it be generalized to Artist? Do you feel Musicians / Artists lack representation in Magic?

In another set, I added actor, artist, and musician as types. Since a card references the musician type, I just thought I'd update this card. I feel those professions are distinct enough to each warrant their own class type (I also used dancer, though I already used dancer in a previous set). Also, currently I anticipate that most creatures in the set would have one of those professions as their class type. Seeing just artist on a third or so of the creatures in the set wouldn't be a good look for the set as a whole imho.