I was thinking about my first wild camps, when I was using synthetic bags - which is what I was using in the early days.
I started off with just a 2 season bag - a Eurohike 200! it was what I had from doing some D of E monitoring, and got re used as it packed reasonably small and wasn't bad at 1.35kg. It actually was ok for the uses it got. I was camping spring and Autumn and especially when the weather was cloudy (so no cold nights) it was comfy and warm. Still have it.
So - just food for thought... no need to go mad on rating if your camping season isn't going too far into cold temps. Also you can wear a primaloft jacket if you do get a bit cold... and heat holder socks are great! I would look at the
Vango Ultralite bags for a decent bag without too much spend (link for example)
My first winter synthetic bag was a Snugpak softie Autumn rated to -5. I used it in the Cairngorms. Wasn't the lightest, but did pack down due to Snugpak's synthetic fill, even if it did feel like I could motty someone with it when it was compressed. Might be worth a look at the Snugpak ones as well.
Always used some sort of reflective silver mat, and an air mattress which helped.
I now have my down bags, but looking back the important thing was I was still able to get out and enjoy my wild camps even though they weren't the lightest (but did pack small enough that I didn't look like a gap year traveller).
I was good at keeping the rest of my gear to a minimum and keeping the weight down of the other items, so the weight penalty of my bag wasn't felt so much. I think my first wild camp pack was 30L. It was a roll top though and I think the person who came up with that figure couldn't add up.
Oh and I sleep cold and always have cold hands.