Looks like you will never get a JSON response on successfully creating an object..
If for example you have a validation on required/unique and you try to create the same object twice, you will get a JSON response.
You can influence the message content by using the validation validation feedback activity, when using a microflow for insert.
Hope this helps.
Not the best ‘work around’ but it might (temporarily) give you some flexibility
Will lead to a response on success
Would need to figure out how to influence the other content.. Unfortunately, the microflow does not allow an object or string as endpoint..