I think there might be a misconception forming so just to clarify, spellcasting classes have spell modifiers depending on the class, for a wizard that would be intelligence. When you check your character stats or under equipment it will show your spell attack and spell DC which is only modified by the ability score your class uses. Then, for example a poison con save would be the target's constitution save vs your spell DC.
You don't need to have every stat be good to get your various spell saves through, just consider if the enemy you're fighting would have a weak save or a strong save against it. Spellcasters typically are weaker on physical saves than mental, and typically fighters have weaker saves vs mental and stronger physical saves.
