I have gotten HQ Yak Milk as well.
However I have to agree with Waeksyn, the drop rates are terrible. My numbers are very similar for Yak Milk farming. Okeanis Eggs were a little bit easier...but the ram meat from Forelands (Lontghorn Chump or whatever) was another ballbuster.
It just seems out of whack when I know I can get at least 6 Heavens Lemon every 30 minutes, and the chances of getting that same number of Yak Milk in 30 minutes is left to RNGesus