Is there any way to make lightning bolts deal more damage?

Commands cannot do anything about lightnings. They are instantaneous events, not entities (@e[type=lightning_bolt] does not work). They are not even saved to file, closing and re-opening the world stops the ~1 second long animation.

The best thing you could do would be to summon lightnings via commands. In that case you can just do whatever you want at their location, like an instant damage effect or just summoning multiple lightnings.