Stacking extra attack

[JackPhoenix]

How do you do that? "You can only cast spells while wearing armor that are from the same class that provided the proficiency"?

Yes. Or better yet, do something useful with the spellcasting components: Spells with somatic components can't be cast while wearing medium or heavy armor or using a shield. From there, you either link components with classes (wizards use somatic components, clerics don't) or type of magic (arcane magic uses somatic components, divine magic doesn't). You're already tracking prepared/known spells separately anyway.

And then you have to come up with some artificial reason why Wizard spells don't work if you're wearing leather armor, but the same spell when cast by an Artificer does. (Or even more glaringly, if you're an Artificer/Wizard multiclass, why the spells coming out of your Wizard spellbook require you to strip down to your skivvies, but the spells in your Artificer formula book don't.)

They are not the same spells. Artificer already require you to use the tool or infused item, and they shouldn't be casting spells in the first place.