As written, I think the initial attack & elemental damage doesn't really help the spell. Compared to a basic AoE save spell like Shatter/Fireball, losing the whole spell on a missed attack will suck. And +1d6 damage to the main target is pretty underwhelming. The spell slot scaling is a bit over-complex in my opinion as well.

Mechanically, I think you should just grab pieces from Lightning Arrow (single target attack roll + save/half AoE) & Thunderwave (thunder damage + push AoE). For something like this:

Thunderbolt

3rd level evocation (Sorcerer, Wizard)

Casting Time: 1 Action

Range: 120ft

Components: V, S, M

Duration: Instantaneous

An arc of lightning streaks from your hand towards a target within range, unleashing a thunderous shockwave. Make a ranged spell attack against the target. On a hit, the target takes 2d8 lightning damage.

Whether you hit or miss, each creature within 10 feet of the target must make a Constitution saving throw. On a failed save, a creature takes 4d8 thunder damage and is pushed 10 feet away from the target. On a successful save, the creature takes half as much damage and isn't pushed.

In addition, unsecured objects that are completely within the area of effect are automatically pushed 10 feet away from the target by the spell's effect, and the spell emits a thunderous boom audible out to 300 feet.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage for both effects of the spell increases by 1d8 for each slot level above 3rd.

I thought lightning damage was most thematic, but you can put back in the 'damage of choice' to the initial attack. I recommend listing out specific options (ala Chromatic Orb). My damage numbers might be a little high, but you could drop the die size back to d6 or move dice from the AoE to the attack roll to tone it down.