Hi caliburn
What you need to achieve is our premium add-on ATUM Product Levels. With this add-on you can create BOMS (product parts or raw materials) and then create the final products and link the BOMS to this product, so its stock will be deducted from the BOMS.
In your case, if you have already products created and you want to link them to variable products, you should convert these products to raw materials or product parts as simple products can be linked. In any case, if these BOMS are sellable you can always enable this option.
I leave you here the link to the documentation where you will find useful info about how it works: https://forum.stockmanagementlabs.com/t/product-levels-documentation
And the link to the demo site in case you want to try the free trial: https://stockmanagementlabs.com/addons/atum-product-levels/
Hope this helps,
Best Regards,
Andrea.