Nick here from Shopify. Great question!
While Shopify does have a discount feature for buy X get Y free, it doesn't have the aspect of it increasing automatically which you are looking to do. You can see the Shopify guide for this here.
I'm thinking it could be worth your time exploring some apps in the Shopify App Store which would have more features for this and could hopefully do what you want and need it to do. Let's take a look at some of these below:
If none of the above apps can automatically apply the discount as it goes higher, your best next bet would be to have something custom made by a Shopify Expert. Shopify Experts know the platform inside and out. While this would be a higher upfront cost, it wouldn't be a monthly payment like some of the apps. You can see the full Expert directory here.
Do you think any of the above would work for you?
All the best, Nick