You’re going to have to input your products no matter what system you use as when a product is chosen, your system will have to know the stock levels, and the only way to do that is to input them in some way.
There are various back end systems out there for stock control and ‘warehousing’ which can integrate with Woo commerce, but every single one of them requires a initial stock take to import.
Your product is the same as any other where e-commerce is concerned. For example, if you were selling t-shirts instead of sausages, you’d still have input the stock levels for all your sizes whether you had 3 sizes, or 30 sizes per one.
What will make it easier though is that with Woo commerce, you can create a product such as a sausage, and add all the weight variations and prices and so on, and then simply duplicate it and only change things like the sausage type and price and any other specifics. This will save you time.
Data input where e-commerce is concerned is always a bit of a grind, especially if you have a lot of products and are starting a fresh.