That’s one of the very standard way of faking shadows for multiple entities. Alternatively, you can pre-bake the shadows to a texture if you want something more accurate.
Just a quick browse on this thread… a small suggestion if you will. During development, keep a perf tracking div on screen, it can be a small one like this. It will keep you from going too far and ensure you are resource-attentive.
Hope it helps, cheers ! 