I don't envy the balance team. The system changes designed around reducing his low elo skew (giving him more early power at the cost of scaling power) are the same system changes that grant more power to support role (strong base power).
Maybe transfer some of his base power from his Q to his E? His E is invaluable for jungling since it's the clear ability while in support it's basically just a bit of burst in your all in.
We don't really have a problem with Amumu support honestly. The main reason we remove a champion from a role is when it is excessively skewed or generally unhealthy in what it does. Amumu doesn't really have those problems, so as long as his winrate isn't excessive (which it currently is) then we have no problem with him existing there.