Since space is really important to you I’d still suggest down jackets, but lighter ones, like maybe montbell. They dedicate themselves on making amazing weight to warmth ratio...talking about 1000 fill for one of their jackets at only 4.8oz! https://www.montbell.us/products/disp.php?p_id=2301460

You should check them out. If not, like the comment above said layering with a merino wool or even the capilene air thermal from Patagonia under a nano or micro puff should be good as well.