What is a low carb non-dairy milk I could have that won't cause cavities?

Not a big concern. Of all the things that can cause cavities, non-dairy milk substitutes is nowhere near the top of the list, especially with the amount and frequency one normally uses them. You can always rinse out, when you can't brush and floss. Eat healthy, avoid sugary, sticky snacks, brush and floss, and see your dentist regularly and you should be fine. Ask your own dentist how you are doing.
None. Any carbohydrate can be a food source for oral bacteria to live on, eat, digest and give off acid that "eats away" /dissolves your tooth creating a cavity.