Sell a product in diferent quanitities that draw from a common Stock.

Hello, new to Squarespace, I have a question/problem regarding stock:

I want to sell cookies, but I'll bake them twice a week in 200 batches. I want to be able to sell them in different boxes/quantities, for example a x4 box, a x8 box and a x16 box. But the idea is that if someone buys a x16 box it should draw from the 200 total, so there would be 184 left, then someone buys x4 then 180 left, and so on.

Currently, I have set things up as 3 different products (could be variants but as far as I explored it's the same) but there is no way to sync them to a common product stock and this will lead to oversells or even worse, make it look like there is no stock of x4 boxes when in reality there is plenty, just listed as x16…

Hope I explained this well since English isn't my native language, right now this is a huge issue that I didn't see coming, hope there is a way to do this since it looks like a rather basic and easy thing any ecommerce service should have.

Thank you.

Thank you Paul for your quick response!
It is not the solution I hoped to get since as you said it's quite pricey, but I'm also glad to know there is at least a solution.

I really hope this feature gets aded because as I said it seems pretty basic.