You should always use a new scope for a new application.
While not necessary for a new window in the same browser I generally use a new scope for a new window to group activities together, maybe even a different xaml file.
Modern UI activities are more powerful and flexible than classic but may take time to learn. Also combined with object repository and unified selectors with fuzzy selectors and computer vision they are more resilient.
I feel like the problems youare having is because you are not using new application scopes. Also you can use wildcards and variables in the window scopes if the urls change slightly.