That would be healthier, but it’s no wonder you were disappointed with the results. The stickiness of the dates would definitely let you lower the fat content, but replacing all of that with water is going to give a very different texture.

To mimic the texture of the saturated fats, you’d do better to use olive oil or the avocado oil you suggested and store the result in the refrigerator. Both of those solidify at refrigerator temperatures the way the saturated fats do at room temperature - canola doesn’t, so that won’t work as well.

Replacing the powdered milk with oats (which would also help a little with gelling the mixture) is good, but don’t forget to add a pinch of salt that is inherent in cow’s milk but the oats are lacking.

You’ll still not be getting the flavor exactly, but those two substitutions should get you a lot closer and a much more similar texture. The walnuts in particular are definitely going to throw you off though. You could reduce the cocoa powder slightly to make up for the extra bitterness, but they would still add a heavier earthy flavor to the mix that people used to milk rather than dark chocolate probably won’t find appealing.