Take a sleeping bag rated for the lowest temperature you will encounter. When it gets hotter, sleep with the zip open, with it as a blanket, or without it at all.

Buy a down bag. My 0C synthetic bag has the same bulk and weight as my -12C down bag. These were both purchased around five years ago and were both mid range models. I don't know know much synthetic bag technology has advanced in the past five years, but I doubt by that much.

I only use mummy bags. They have the best warmth to weight ratio compared to square bags or quilts. The hood makes a huge difference to warmth. Of course if you are only camping at above freezing temperatures, then it doesn't matter too much. A hat will do at such mild temperatures. Buy a waterproof compression sack for your sleeping bag. If your sleeping bag gets wet, you will be miserable.

I'd think about using an inflating mattress as opposed to a foam pad. You'll be able to keep it inside your panner as opposed to on top of your rack. Not a huge deal but something to consider.