This didn’t work, not because I was there ( he was the same when others tried to support her), but because he truly believes he can do no wrong
Thing is, OP, you can't make someone else be a good parent.
It would be great it he were a better dad, but he is who he is. If he doesn't want your help in supporting the DC or doesn't do what's in their best interests, you cannot make him do it (unless things get extreme and it becomes a police/SS matter, obviously, but even then, you personally don't have the power to control him).
The only control you have in this situation is damage limitation when the DC come home or get upset by him, and revisiting contact arrangements (if you feel something significant has changed since the last contact order).
In terms of withdrawing contact, do you have clear reasons for wanting to do this now over and above what has already been heard by the court, or is it more the frustration that the court didn't prioritise what you see as the risks? The fact DD has a MH crisis isn't an argument for withdrawing contact in itself, but his handling of it might be depending on what's actually happening. Are you taking notes of all the incidents?
As an aside - in the same way that you have no power over him and his parenting, he only has power over you if you give it to him. I say this as someone who has felt exactly the same way as you over the years - it's awful feeling completely powerless to protect your DC. But you can protect yourself from him and this will make you stronger.
How old are the DC? Do they want to go to contact?