One other thing. Withbivvy sacks, go minimalist. Stay away from those with poles and other frou=frou. Those are basically tents, and incur additional weight.
Classical bivvys were developed to servee as minimalist shields for technical climbers performing overnight and multi-day ascents.
Light is right (up to a point - It can bee downright silly if carried too far
one or more gifts or other benefits from the co-op.