This is a 2 teir idea.
1st - All products should automatically generate related products based on descriptions, categories, titles, etc. but ONLY when you have not manually entered product(s). As soon as you manually enter a product this auto-fill option should go away. (Could just be a toggled option)
2nd (and more importantly) - When you manually add a related product (P2) to a product (P1), then P1 should get automatically added as a related product to P2. Since we have already dictated that they are related we shouldn't have to enter this into every product.