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.

Feels a little bit overtuned IMO?

Consider that the UA version of the storm sorc has a levelled spell list that you could substitute for the character.