Jazz Double Synthetic Sleeping Bag

You say your clothes are optimized… but Nemo jazz is a 30deg bag and I’d be shocked if you tried a zero deg quilt with the right layers and couldn’t make it work. I find my 0deg quilt plus alpha fleece and wearing my (dry) rain gear is good well below freezing even cowboy camping with a breeze. But you gotta practice and iterate. TLDR: the bags/quilts you tried might not be your problem.

Since it’s temp controlled, I think any sleeping bag will work. I think it just depends on your budget. I have a Nemo jazz that I use for regular camping. When we sleep in the car- we just use a sheet and a blanket. Good luck op.

I believe the NEMO jazz does. Amazing quality, will last a long time, and made to be a wonderful luxurious bag

My 30° Nemo Jazz on a cot with a Thermarest Luxury Map XL keeps me super warm in those temps with just a merino base layer.

This. I hate mummy bags. I move too much in my sleep. My Nemo takes up a bit more space in my pack but it is so comfortable and a good night’s sleep, where I can fully roll around in my bag, is worth it. I will never go back to a mummy bag.

It might depend on how you're built, but I can get my leg up like that inside my Nemo bag. I have short legs, though.

I really wanted to love the Nemo, but if OP is a smaller person, Id warn they have too much dead space.