If your slow cooker does not have a saute function, saute the ingredients in a skillet and then transfer to the slow cooker to proceed. Do not skip the saute method. It is vital in creating a robust and rich flavor.
If you don't want to use oat milk, use any unsweetened, neutral-flavored, full-bodied vegan milk. Rice milk is not a good substitute. Flax milk would be a good substitute.