Get one of the larger WM bags like the badger or a sequoia, then get a really comfortable mattress like a large x therm or a sea to summit comfort plus.
I use a 10 degree feathered friends something or other for everything (live in AK). It’s oversized, so loses some efficiency, but it is comfortable if I need to layer up with down inside. I rarely camp when it’s stupid cold anymore (well, RTT with diesel heater I’ll camp anytime), but this has been fine down to around zero in my tipi.
I recently got a Cumulus bag. I was able to customize it the way I wanted in regards to fill, width, length and material. Price was great and the bag is super comfortable.
Seem the recommendation previously but don't skimp on the pad below you. It will make a world of difference in your sleep.