Damaging spells don't scale that much better than just upcasting 3rd lvl spells, except for Meteor Swarm at 9th lvl, so if you just wanna do good AoE damage, stacking multiple boni on your damage roll will be about as good, before character lvl 15, when you could be a Blood Cleric 1/Evoker14 and maximize your damage spells.
These are the straight bonus I can remember from a quick reminder of the subclasses names, there may be some others I don't remember:
- Artillerist Artificer, lvl 5 +1d8 damage (requires specific focus)
- Spirits Bard, lvl 3 +1d6 damage (requires specific focus)
- Wildfire Druid, lvl 6 +1d8 damage (fire only)
- Draconic Sorcerer, lvl 6 +Cha to damage (1 element)