Some objects in Octopus provide way to find the places that they are referenced. For example Certificates and Account both provide a usage field, and that information is also available via the API.
For example, the screenshot below shows the usages of a certificate as part of a deployment target.
However to report the relationships between things like projects, variables etc, the API that you have referenced is the best option. The API gives access to all entities in Octopus, so to generate a custom report this is the best solution.