I think canned lite coconut milk works best. It's not so thick, such as full-fat, that it will weigh the potatoes down. And don't worry - the potatoes, miso, vegan butter, and roasted garlic are the flavors you taste, not coconut. If you can't find lite coconut milk, you can make your own by mixing together 1 part full-fat coconut milk with 2 parts water. If you are allergic to coconut milk, use full-fat oat milk, like this one from Oatly (you might need a couple extra tablespoons). I don't recommend soy milk, which some recipes use, because it has a slightly sweet taste (even unsweetened varieties) that is weird in savory food. And I certainly don't recommend a thin plant milk such as almond milk. Mashed potatoes are meant to be indulgent, so you need a creamy substitute for the classic half and half / heavy cream / whole milk.