I’ve lived in WA state for 30+ years. I have always used a down bag/quilt with absolutely no problems, all four seasons. There is just no way that a synthetic bag/quilt warm enough for winter (say comfort rated to 20 degF ) is going to be light weight and small enough (packed size) compared to down.
I have used a Western mountaineering bag rated to 25 in nights down to 16degF with no problem because: 1) in the tent is is about 5 degrees F warmer than outside and 2) I wore either some extra clothes in the sleeping bag, or used a Sea to summit sleeping bag liner (their lightest one). 3) I sleep right next to my wife, and I think that makes one side of my bag/quilt not as exposed to the cold.
I keep my down clothing and bag in a waterproof pack liner. Got to keep it safe and dry.
I think a 20 degree comfort rating is sufficient for bag (and/or quilt), enhanced, if needed (for really cold nights, especially in the wee hours of the morning before the sun comes up), with extra clothing. The sea to summit bag liner is nice when it is down to 15 deg F or so (it weights about 1/2 Lb).
I currently use a nunatak quilt , 20 degree rating (with 2 oz extra fill). No problems. And it being a quilt (instead of a bag) has not posed any problems.
Good luck!