Lost Ranger 15°

I have a Big Agnes lost ranger UL which is a 2 part system, one of which is like you describe, the other a regular bag, or you combine both for maximum insulation. 

Baselayers!!! Midlayers!! Outerlayer! Wool socks. Baselayer as a Dry suit for sleeping Thermal Sleeping pad. Big Agnes Lost Ranger 0F Sleeping Bag $$ you are going to spend some cash money. It's like crack.

Big Agnes Lost Ranger 0F Sleeping Bag and Pad. I can adjust accordingly with this bag because its two bags in one really. You specified camping and this is a great camping bag. I got it cheap a couple of springs ago. I like it.

We have a Big Agnes something or other 15 degree down bag we bought for around $300...10 years ago. It's great, although more like a 30 degree bag. Size / weight is about equal to two sleeping bags. We also tried out Feathered Friends' Spoonbill which is well out of your price range but might be interesting to look at to see how an actually warm two person bag looks.

 So the problem with any normal two-person sleeping bag is that there's an air gap between you if you don't sleep snuggled up.  The Big Agnes we have has a small flap that covers that up. We have also stuffed clothes in between us when it was colder. That works for 15 to 20°, but not colder. The reason I said the spoonbill was interesting is that it has individual draft hoods for each person so you share warmth but actually can't cuddle. 

These were all over the industry a couple of years ago, but have fallen out of favour, and I actually can't find any current single-bag, adult sized offerings by a major company that are made this way. The Rab Mythic Ultra 120 "modular system" is now discontinued, I believe, but would be the most UL uninsulated bottom option. The BA Lost Ranger is an actual modular system consisting of two bags, a fully insulated mummy bag and a backless/pad sleeve style outer rectangular bag

I would go with a quilt, they are great for moving around. Zenbivy is most peoples favorite, I really enjoy my Big Agnes.

If your biggest concern about a sleeping bag is room for your legs to move without feeling constricted, I would try out Nemo or Big Agnes sleeping bags. Both tend to have wider profiles. Definitely go for a bag that is rated at least 10 degrees lower - preferably lower - than the temps you expect to sleep in. Theres multiple problems with layering with clothing like this, one of which is not the warmth specific factor but the breathability. One or two layers is one thing, but with all the listed clothing and then something like the picture on top, you’re going to build up humidity near your body you can’t get rid of, and you’ll become even colder. Next, a sleeping bag basically creates a warm pocket of air around you. The insulation in the bag helps create and maintain this pocket. The more compressed this insulation, the worse it performs. With layering clothing, you compress the insulation, making it less effective. On top of that, insulation is minimal if not nonexistent at the seams. Some puffy jackets and “extreme temp” clothing gets around this by minimizing seams but there’s almost always a few somewhere, which means no air pockets and no warmth. If I could recommend the best case scenario for you, it would be this: find a sleeping bag you can tolerate rated to a temp appropriate for your conditions, 15F at a minimum, best scenario a 0F. Dress in only your smartwool layers, and make sure you have a good pad/sleep set up underneath you. Sources: avid outdoorsmen, backpacker in sub 20 degree temps, and former REI employee who specialized in camping gear.

A Big Agnes sleeping bag with down that fit me correctly. I was iffy on the price at first, but it turns out I had basically used ill fitting, non insulated sleeping bags for years of wilderness camping. Getting a good nights sleep anywhere, especially away from home, makes a huge difference.