If you haven’t already definitely add a cull distance volume:
Cull Distance Volumes in Unreal Engine | Unreal Engine 5.6 Documentation | Epic Developer Community
For the window materials - if all the windows of the same type get dark at the same time, they can all share the same runtime material instance, which would be better for performance).
If all windows of the same type need to change at different times, then each would have to have its own runtime material instance, which would be lower performance.