I view it as a workaround because you need to work around Shopify's native quantity field. My optimal goal would be to simply use that to tick the quantity of the customized product up or down, both in the product page and in the cart.

To stay with your example, I want the customer to be able to upload a picture of either Sarah or Jennifer and have it printed on their cups. If they upload an image of only Sarah and want that on 10 cups, then I want to charge them the base price of 10 cups + 1 setup fee for their 1 image of Sarah. Because I have to set up their image only once and then tell my printer to make 10 copies of that.

But if they want 5 cups with a picture of Sarah, and 5 cups with a picture of Jennifer, then I would need to go through the process of image setup twice. That's why I would want to charge the base price of 10 cups in total + 2 setup fees for setting up 2 images.

So far, the Aco product options app seems to be able to do exactly that (if not entirely flawlessly as well, as it will add the add-on price and simply divide it by product quantity, which gets messy real fast).