@rafa I don't know about the specific problem, but my experience with SwiftUI is that when you try to use Apple’s "official” views, you will find many roadblocks and limitations; Still, you can build nearly everything pixel-perfect when you create your views from scratch.
So, my advice would be to not use a Toolbar but just a regular, custom SwiftUI view.
@rafa@davidbures from memory that would make it work like a HStack, so yeah centered if you place it between two Spacers. Wait, you want a single button to fill the entire width of the Toolbar?