Do you need to control the entire response or just ensure that the response has “at least” the variables you require?
Look at the rest api for Start Instance and there is a “withVariablesInReturn” param you can have the created instance return all of the created variables.
Other options as you will see in the forum are to create a new web service with something like jax-ws. Or create a small middleware app that fronts the camunda API and will manipulate your data into its own form. If you decide middleware, here is a example: Function as a Service API