Drop the bivy. Keep the space blanket.
You already have a sleep setup. The space blanket gives redundancy in several areas. In first aid to prevent hypothermia of a casualty. In hiking to boost temperature ratings when cold and protection from the sun when hot. It's also useful as protection against wind and water as tarp or poncho.
As a survival item the space blanket has a yes in my book. It's light and cheap. Has several uses. Can be used and reused, or given away to someone in need.
I also carry two fire starters, two knives , two lights and rations for one extra day. And a space blanket