We need assemblies, not composite items. We make things, and need to track which parts purchased from who went into the item. We do not want a new inventory item each time it is made ( we would have millions of items in the inventory list we don't need) we just need to track which part went into which item.

Your current batch/serial system is useless for this, because it works on bills and invoices. We need it to represent real parts in Inventory. I thought that was why this was called "Inventory". If it were the case that I knew a batch or lot number only when the bill arrived I would do it in Books. Unfortunately, the inventory arrives to a shop floor that does not see the invoice or the bill. They get a part, that part has a lot number on it, and it gets assembled into a product that we sell. We need to know what Products contain what parts, with what lots - sometimes years later at the customers request.