If you specify the "f" parameter in your request, you should be able to get your desired aggregated output. You can do this with both jobs/${search_id}/results as well as jobs/export. Below is a small code snippet for both of these calls.
curl -k -u admin:changeme https://localhost:8089/services/search/jobs/export -d search="search index%3D_internal | top limit%3D10 sourcetype" -d output_mode=csv -d f=sourcetype -d f=count
OR
curl -k -u admin:changeme https://localhost:8089/services/search/jobs/${search_id]/results --get -d output_mode=csv -d f=sourcetype -d f=count
Also, I am not sure if you are aware of the Splunk SDKs (Java, JavaScript, Python, PHP) but I would encourage accessing the REST APIs through them as they make it easier for you to manage and work with Splunk objects and you can stay in the comfort zone of your preferred language.
If you need help or code sample to implement the commands above through any of the SDKs, feel free to contact me.
Hope this helps.
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