I think just sticking to 3d6 lightning on a failed save and half as much on a failed save is fine rather than a combo of 3d6 lightning and 2d6 thunder.
The spell seems pretty powerful before the dismissal effect - especially if used at the beginning of combat, which would be optimal.
Good.
Quality content is better to me than having a large quantity of content. I’d rather finish a game and think, “wow, that was solid” instead of “wow, when will this end”. Even if it’s endgame content; I don’t want it to feel like chores.