The thing that gets me is that the price of oat milk is so completely divorced from what it cost to produce. It's absolutely wild that oat milk, made with one of the cheapest bulk grains, is as expensive as almond milk, made with an expensive nut.
The only exception is soymilk which does actually have a lot of protein and also has a really long tradition. People have been making soymilk for over 2,000 years. American grocery store soymilk is quite different but you can actually go to an Asian grocery store and still find soymilk that has a single ingredient.
Here in Europe we can fin the single ingredients soy milk if you look down the shelf: the cheapest brand and groceries have no additives while the fanciest ones are full of thickeners and various sugars. I recommend the store’ brand from Carrefour, Lidl and Leclerc.