I think Thucy should stay as he is cause he is a junior Version of Cianna, and can help smaller players to beat heroic bosses that requires stun.
Shayd should also stay as she is. Queen Brynhildr is not a better Version of Shayd she is simply another kind of Mage.

Shayd can stuns one Target but deal normal DMG.
Queen Brynhildr can stun all Targets but does more or less no DMG.

And I don't see how your changes would make a difference, at the end there would still be 3 stunners and the big guys does the big DMG cause they have 2 more hero's that will make much more DMG then the smaller.

Btw Time is no Factor, the Percentage wont change. All have to do more Rounds, not only the small Players.