Garthanos
Arcadian Knight
I remember mcing oldschool to be very restrictive feeling? 3e was simple and freeform but very abusable as you have said.The advantage of it though is the old scgool MCing which modern D&D has struggled with since 3E.
However it looks like 5e fixed 3e multi-classing by removing the front loading factor of classes.
4e used arguably 2 complex methods of multiclassing one is a dabbling multiclassing bound to the feat system. Which is pretty flexible and can be engaged throughout the characters career but always feels like dabbling and a more convoluted hybriding that one has to decide on when you design the character,
Using both gives a huge amounts of variation but isn't anything close to elegance.