For a private dairy
You buy and you sell. The margin between them is the business, so it should not take an evening to find.
Milk bought from suppliers and milk sold to customers are recorded on the same screen, kept apart in the accounts, and brought together in the earnings view: total bought, total sold, average buying rate, average selling rate, margin per litre, and the profit.
Goods go on the same bill. Ghee, paneer, feed — anything you sell a member is added to their account and subtracted at settlement, with stock coming down as you sell.
You can see who matters: the top five suppliers and top five customers by volume for whatever period you pick.
One honest limit. If any product sale in the period is missing its cost price, the product profit is not shown at all rather than shown wrongly. A blank is a question; a wrong profit is a decision made on a lie.