add them to a Collection (preferably a "custom type" called something else)
set a discount for the entire collection (% or fixed), or for each individual item
if a customer buys the entire collection, they get the discount
I'm new to Shopify so I'm not familiar with all the APIs, but I saw https://help.shopify.com/en/api/reference/metafield which could be used to set the discounts, but I'm unsure of how to do the product selection (unless I really would just have to use Collections—which is fine), and I'm very unsure of how to actually apply the discount when the customer checks out.
I'd just make each of these a collection with a private coupon applied to it, but I have to limit customers to just one coupon per transaction. Maybe this is an easier problem to solve?