Entirely compostable I don’t know. But I have a recycle wool shirt from Patagonia that is the best.

Depending from where your winter is you’ll need some synthetic fiber in your coat so it don’t get all wet. However there’s some wool coat but forget playing in the snow with those.

Patagonia makes good lightweight synthetic winter wear, no animal products necessary. A micro-puff and an r1 air fleece will cover you well below freezing.

For all clothing I default to thrift stores. For me, better consumption is buying stuff that already exists as a way to reduce demand. Also mending and taking care of what I have.

If you have to buy new, I'm not sure compostable is the "right" choice. Better to buy something that will last a long time if you're buying new, and compostable is likely to degrade (in my assumption).