Hello,
I’m not sure if you still need help here, but I’d like to give a starting point to you either way - At least for other people who are searching. Even if fixtures aren’t the right thing per definition, we can achieve this by using commands in a similar way I think - as we use the Shopware API to create test data = similar approach.
In our order fixture, we do a complete checkout API-wise which means we have to add products to the cart as interim stage. Maybe you can write an own command based on this fixture as a starting point.
Kind regards!
Ramona