There's no debating that milk is heavy to digest, more so than water infused with whatever you want (tea, rice, soy, oat, almond...), whatever your ancestry. You actually need some enzyme to cut it down so that you can metabolize it. This process doesn't exist for infused water.
> The solution then is to just not drink milk, not substitute it with oat-canola slurry.
No, you can buy very fine "vegetable milks", just not oatly. I don't have any experience in US but in every west-european supermarket you can find heaps of them in the organic section. And if you wanna argue about the price: just make it yourself, it's dirt-cheap, just buy bulk cereals/beans (oat, soy, rice, chickpea ... be creative it works with mostly anything), boil it in water, mix it and drain it. Ta-Da, you just got pure vegetable milk for pennies and as bonus heaps of dry draining residue you can use as wheat replacement in cakes or croquettes (or used as semolina).