Need to change external checkout link based on quantity selector Shopify

I have an external checkout but still want to use shopify for my storefront.


As you can see, there are different quantities that can be bought in packs. I have created 4 different checkouts for each amount on the selector.

Is there a way to change 'add to cart' link based on what quantity is selected?

There is no out-of-the-box solution for this, as Shopify's checkout process is dynamically coded.
You need to use hooks to make this happen using custom coding.