Some questions are hard, David. Some questions are awkward. Some are simply unanswerable.
Men's Down Sweater
Yours, on the other hand, is easy. What you need, and all you need, is a down sweater. NOTHING has more value in terms of warmth for weight, comfort, and utility than a down sweater. Compresses to near pocket-size, and good for light recreation into the teens.
The odd thing is that not too many companies make this most useful of all garments. I can cite two, although I might be missing some outliers. The gold standard of down sweaters, in my view, is the Patagonia Mens Down Sweater ($200). Just a beautiful piece of gearsuper-light (12 ounces), remarkably warm, and compresses to the size of a baseball.
Even ligher, MontBells Ex Light Down Jacket ($160), which weighs in under six ouncesthats about the weight of a heavy pair of socks. And very close to the Patagonias in fill volume, so very close in warmth. Fantastic deal.
Synthetic...well, it has its advantagesmainly warmer when soaking wet. But if it is cold, its typically not that wet, yes? That said, there are some nice pieces out there. Integral Designs makes an excellent light sweater with Primaloft insulation for $160 called called the PLQ Jacket. It only weighs 12 ounces in size medium, which is very respectable.
So there you go. Case closed.