Low and slow is definitely the way to go here. If you stir too much or cook the milk too quickly, you won't get those fantastic thick layers of cream in the dish. You do have the option to just thicken and scrape the milk solids from the sides, like in this rabdi that I have shown in my Shahi Tukda recipe. Again, there won't be the same layers but you'll still have a delicious dessert! There is a method of making instant rabdi with condensed milk. But it does not have an authentic taste or consistency like this traditional lachha rabdi recipe.