I suppose the closest thing to what might be called conditionally optimal would be the milk of mares and donkeys. They are not ruminants but, like us, have a single-chambered stomach. Their protein is only 40-50% of casein as in humans, unlike cows, goats and sheep which have 80-85% of protein is casein (and in absolute numbers it is 2-3 times less than in ruminants, although still 2 times more than in humans). They also have 5 times higher amount of lactoferrin than ruminants (which is at the level of human milk) and 6 times higher amount of lysozyme.