Product Combination Discount in Commerce 2?
Indeed, the way to go is setting up a module or plugin for this sort of thing. While I did get that working in practice, I still have questions regarding how one can loop through and compare using the Craft CMS 3 Commerce 2 APIs to actually apply the discount based on custom fields on the variants. That will be a separate question entirely.

If you stumble upon this issue, the starting point I used was this;
https://github.com/oof-bar/commerce-example-adjuster

Here's a link to the new question