Hello Camingo,
I exactly have the same questions.
In theory and to be more efficient, sustainability should be integrated at every stage of the process, from the concept to the production and even further (recycling…)
But most of the time, it is either done during the production stage (change of materials) and sometime not even by the designer himself but by some ingineer.
One way to resolve that would be for the design manager to help designers focus on the matter by both talking about it with the client and changing the way the process is managed.
I am doing a surveyon that matter here : ECO-DESIGN MANAGEMENT SURVEY, so feel free to contribute if you are interested!