I'll update this post once I go anywhere with it

This feedback sounds promising.

I am becoming more curious around another description for a similar use case:
An associative table can be created. QStandardItem objects can be generated then based on table entries. Which ways would you like to recommend for the connection of entities to their relationship data?

Unless you go into very specific obscure cases

Another software design possibility would be to fiddle with proxies for data models (and their items), wouldn't it?