Torchlight EXP 0°

I'm gonna ignore the 'cheap' part. Sounds like you want a 0F down mummy bag and a inflatable pad like a Nemo Tensor. I would definitely opt for a 'water-proof' down. the normal price for the pad is $200. a bag like that is can be about $700 but the Big Agnes Torchlight EXP 0 is usually under $400

yes, that is a lot of money. but it is about as light as you are going to get and still be comfy at 10F.

I spend significant time in the field and also western hunt all fall/ winter. I run a Big Agnes Torchlight 0 degree bag and a sea to summit reactor extreme sleeping bag liner when needed for additional warmth. Consistently comfortable in damp, very cold, and snowy conditions. Synthetic fill retains warmth even when wet and dries quickly. I also use my issue bivy when needed.

The Nemo spoon shaped bags, TR Boost or BA Torchlight are all nice and accommodating for flamingo type sleepers (I am one as well.) Also consider a quilt!

Since it's currently at full price, the Magma is about $100 more expensive than either of OP's choices. The big difference is that has 800 fill power down, where the other two are 600 and 650 fill power. My daughter recently replaced a synthetic bag she bought while in college with an REI model, and liked their new sizing system. I think she may have bought the REI Magma. I have and like a Big Agnes 650 down bag I've had for years, but at a 15 degrees F rating, it hasn't gotten a lot of use in recent years. I agree with u/TheBimpo that, unless they are trying to put a full set of gear into a pack of 50 liters or less, OP will do fine with either of their choices.

The older I get the thicker the sleeping pad... Where I live, I need 3 distinct sleeping bags - warm weather (Big Agnes), shoulder seasons/early winter (Nemo), and extreme winter (Big Agnes - takes up the whole backpack!) I have an REI half dome tent - big enough for weekend camping, small enough that I could backpack with it if I'm not using a hammock. I also have several Coleman Sun Domes as lenders, and my boys each have a Kelty when they don't hammock. For front country camping, a nice comfy camp chair is worth its weight... consider a lightweight backpacking chair as an addition or alternative. You can enjoy a lightweight camping chair in camp, but hard to take an XXL rocking camp chair backpacking... Consider hammocking, too, but that's a whole additional rabbit hole to travel down.