Data Relationship from "Assets" to "Products" - help please!

There are really two components to your request. The first is the metadata association / relationships. This should be straightforward to setup using relationship mappings based on the product id or some other key as you have already described.

The tricky part on Bubble is handling files, particularly when you are dealing with them in bulk. We explored various solutions within Bubble but none of them fully satisfied our applications requirements (or they were too much trouble to be worth the implementation inside of Bubble).

However, it is possible to handle this logic strictly within Bubble if you are willing to invest a significant amount of time building out your own solution. The first place to start regardless of storing the files on Bubble or off platform is definitely this plugin:

The author @jonah.deleseleuc is very helpful and the plugin is highly useful for handling bulk uploads (For example using the plugin you can prefix the file name with the product id).