1st, I’ve not used any mod beyond graphics change.
But! Any (pre-winter, still growing) plant material in winter is brittle. Spruce, pine, birch, alder, rowan, willow, juniper. Doesn’t matter. Unless going to further technology and have full on cellulose production... what’s ~1,000 years here? LOL
If anything, my suggestion would be to add recipe to “unravel woven cloth” to remove very carefully and slowly threads from linen and nettle fabrics. And the resulting product/thread should be *no quality* item.
I’m surprised no one has mentioned (not sure it’s in any mod either) is tendon sinew. That has historically been widely used material for strong & thin cordage. Just need to add that as “by-product” when butchering any sizable quarry.