The milk tea here is quite a bit stronger than you’d make for a regular bubble tea, because the oatmeal dilutes the tea flavor even more than the milk (or non-dairy creamer) does. Feel free to adjust to your taste. I borrowed an old tip from Southern sweet tea in reducing the sometimes puckery bitterness from strong tea — a pinch of baking soda is all it takes.
To make the milk tea with non-dairy powdered creamer or powdered milk, use 2 cups water instead of 1 1/2 cups, and add 3-4 tbsp powdered creamer or milk, plus 1-2 tbsp sugar (or as much as you’d like, to taste).
As for the black tea, I find that this works with most any black tea you prefer. Ten Ren is a solid brand and recommends a black tea blend specifically for bubble tea; Pu-Erh, Ceylon, Assam, or English Breakfast would all be great, too, and even Lipton is just fine.