Having survived a night at -40 in the open at altitude in a light sleeping bag and snowmobile suit, I now have the attitude of carrying MORE vs less sleeping bag. I HATE to be cold in my sleeping bag. So anyway this is what I do, YMMV: WM Super Kodiak

Getting into my bag is a process. First I may need to get in with everything I have on and wait to warm up. I basically prefer to sleep completely unencumbered at home (Read nude), but in a tent I do wear a light pair of insulated long underwear and a fleece jacket and sleeping gloves and balaclava. thats all

I then get out and pull off my down bibs and tuck them under my head, then I have to get the underpants off and the longunderwear back on which generally means another dive into the bag to warm up. Then its time to remove the big coat and strip down to a thin warm layer on top and then the final dive into the bag. no socks... A quality sleeping bag should keep your legs and feet toasty warm without socks. In fact if your feet are cold in the footsak of a mummy bag it means the blood getting to your feet is cold: Insulate your legs more, but put your big down jacket over your sleeping bag in extreme cold to insulate your torso. For that matter a cord tied lightly around your sleeping bag say just above your knees, will make your bag warmer by dividing it up into compartments that closes off drafts inside your sleeping bag. been there, done that...
The only requirement for my system to work is a pee bottle. If I had to get out of the tent in the night I would wear the down bibs to bed, then reach for my big coat, put it on, dive into the bag and when I was warm I would get out and put my boots on and leave the tent.

I'm not certain that thick leg insulation as in pants or bibs, actually makes you warm because your legs can't warm each other up. I'd rather pull the down bibs into the bag to take up air space and add insulation, say behind my back, especially in extreme winds.
Jim