Hey, there.
Dallas here from the Shopify Social Care Team.
Thank you for adding those screenshots. It really helps me to have a first-hand understanding of what you’re wanting.
Based on my understanding of what you’re needing I think you should look at the app Infinite Options. It’s one of the best apps for variant customizations and it allows you to build a variety of different features for the variants of your product page. If you were to use this app then the inventory would be adjusted automatically whenever a product is purchased.
Have you looked over that option yet?
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If that option is still not what you’re looking for then I suggest you try to get this custom built. The Shopify Experts are very talented when it comes to building custom features for websites. What you’re looking for sounds like a pretty simple concept so I don’t think it would be out of their capabilities.
Please let me know if I am misunderstanding what you’re looking to do. Also, if you have any further questions please don’t hesitate to let me know!