just one other option maybe folks haven't considered
I carry a light 50 degree Apex quilt, replete with poncho hole. I carry it while hunting for glassing (or an emergency)- it weighs a svelte 12 oz. For glassing it adds a substantial amount of warmth, on it's own for sleeping it's not overly warm (have had it down to 40 a couple of times and wasn't terrible, had it down to freezing and it sucked! )
BUT added to another bag when backpacking, it adds quite a bit of warmth AND being synthetic and layered on the outside it helps keep down drier (and loftier)
anyways for 12 oz, small volume and multi-functional, I've been happy to pack it
This is basically the system I use now. Just need to find a better way to keep the top quilt from constantly slipping off. I used straps underneath, but it wasnt 100%