Found this on BG3 wiki
https://bg3.wiki/wiki/Spells"In the case of Multiclassing, learned class spells will use the Spellcasting Ability Modifier for the class the spell is learned by. So a Fire Bolt learned by a Wizard will use INT for its attack rolls, but a Fire Bolt learned by a Sorcerer will use CHA for its attack rolls."
If this is true then JandK was right and I may end up needing Charisma after all if I want to use damage cantrips. In which I'm gonna have to work on my build a bit more.
Also apparently Firebolt does eventually deal 3d10 damage according to wiki. Didn't do that much as a Wizard tho. My game must've been bugged.
https://bg3.wiki/wiki/Fire_Bolt