Stress testing an API endpoint with a callback mechanism

I have a RESTapi I'm developing that has an endpoint that returns a uuid and starts a background task that eventually returns some data via a POST request to a callback url on the client.

I want to somehow use the body of the POST request to trigger another request and then maybe repeat this again to form a chain of requests. More specifically, I want something that can mock a client server by receiving the POST request but then use the body of the POST request to the mock server to send another request to the API.

So the first question is how can this be achieved? I've tried Postman's mock server and Webhook tools, but none of them quite work.

Second question is how would I do this with lots of chains of requests acting concurrently?