This is way too clunky compared to the normal rules, and I'm not sure any of this is needed. You don't need to be so afraid of multiclassing. There are always trade-offs when you multiclass, and those characters aren't significantly more powerful than straight classes. Usually they can do something really well such as smites or quickened eldritch blasts, but there are costs associated with multiclassing that in play are usually more pronounced than they seem on here, especially being behind in spell level for casters. I'd just allow multiclassing and enjoy the ride.