I don't really see this as a downside to the German legislation. If anything it's ... good? It gives the employee a ton of control over whether they do overtime or not. Yes, it's a bit weird that it's technically illegal... but if neither side complains then it's fine?

The power imbalance is so high that normally any scenario where you can 'agree' to work overtime usually leads to employees always having to technically consent or face termination.

It's not perfect but I don't know that there is a better solution given the power imbalance and the game theory of these situations.