You seem to have a good grasp on what you are comfortable with. For what it is worth here is my perspective. I was fairly sure i could get by with one bag for winter, spring and fall. I used weight as a criteria. I felt a good quality bag with 800 or 850 fill down that was less than 2.2 pounds (kilo) would be sufficient. I purchased the NF Hightailit 3S with a comfort rating at 27F. It weighs exactly 32 ounces, has tons of loft and has worked well. I was comfortably warm at 30F with just a t-shirt and underpants. I'm sure it will be perfectly fine at 27F with long johns and a base layer top. That said, I consider the manufacturer's temp ratings as only one of many factors when purchasing a new sleeping bag.
Jim M