I've recently run into an issue fetching nested data within a content relationship field.
It was due to not being able to fetch all the RichText field in a fetch link query, so I had to use a Graph Query instead.
The problem I am facing is that there is no way (at least I don't think there is), to fetch the rest of the slices with something like ...slices.
Here you can see I have fetched the required nested content in a slice, but now it will only return this one slice.
For now, I have done a temporary solution and added in all the other slices manually.
Welcome to the Prismic community, and thanks for your feedback.
For the moment, you need to specify each slice name to retrieve them and retrieve different fields related to another document; you need to specify the content relationship name too. I agree with your feedback and added it as a product feature idea, but It's not on our roadmap for the moment.