Lightning based blaster build?

[Psyren]

Eh? What are you talking about? Scribe Wizards have a way to turn spells into lightning damage very easily at 9th level. The wording for their ability is "When you cast a wizard spell with a spell slot, you can temporarily replace its damage type with a type that appears in another spell in your spellbook, which magically alters the spell's formula for this casting only."

So you have to have a spell that has the required damage type. Prismatic Wall is a 9th level spell that deals Fire, Acid, Lightning, Poison, and Cold damage, that satisfies the requirement for Scribe Wizards. Its kind of like how Storm Sphere grants Lightning or Bludgeoning damage. I will admit, the Scribe Wizard does lack thunder damage at high levels. But that's not a major issue.

Indeed - Scribes can get Electric Meteors just fine, and proceed to maximize them with a Destructive Wrath multiclass.

Draconic sorcerers only advantage is 5ish damage to a single target when casting an applicable spell.

It is pretty lackluster as a blasting option.

Elemental Affinity adds the bonus to one damage roll, not one target. That distinction matters, because spells where you make a single damage roll and apply the result to multiple targets - such as, say, the vast majority of AoE damage spells, or spells like Magic Missile where you roll once and multiply - will effectively multiply the damage bonus accordingly.

For example - when your Bronze Draconic Sorcerer fries 3 ogres with a lightning bolt, the 8d6 damage is a single damage roll which you then apply your Charisma modifier to - which ultimately means each of the three ogres is also getting +Cha to damage. If your Charisma bonus is +5, the total damage bonus you gained from that feature for that casting is therefore +15, not +5.