Here's a little known trick. Workspace uses API 2.0; and they provide a "debugger option".
If you build out a report similar to what you need in Workspace, then go to Help > Enable Debugger a new icon will appear on each visualization that looks like a little bug.
When you click on that icon, it will bring up a popup that will show you one or more timestamps (some tables will require multiple API calls). You can then click on each of those timestamps to open up a page which will show you the API calls used in XML, curl and JSON.
You can actually copy and use these API calls yourself to retrieve the data.
If you can't get exactly what you want in Workspace, this will still be helpful for you to see how the requests are formed, and use them as a base to tweak in order to achieve your desired API request.
Fundamentally, API 2.0 does require multiple calls for breakdown data, this was done to make the API perform better.
In the old API 1.4, you could do breakdowns, but as you probably know, you had to send the request into a queue, then you had to keep checking the queue for the report to be done, then you could request the report....
In API 2.0, there is no longer the concept of the queue, requests are made and returned directly... so to make sure things run smoothly, they require breakdowns to be run separately.