mitmproxy (https://mitmproxy.org/) has scripting support that will let you do most of this.

For example, you can expose mitmproxy, listen to HTTP requests for a specific host (using this API: https://docs.mitmproxy.org/stable/api/mitmproxy/http.html), intercept the request, do whatever API calls you need, and inject a response without ever forwarding the request to the original server.

Alternatively, you could modify the request and then change the request destination, like in this example here: https://docs.mitmproxy.org/stable/addons-examples/#http-redi.... Using the WSGI support, you could even use normal Python annotations to build your own API without doing too much pattern matching: https://docs.mitmproxy.org/stable/addons-examples/#wsgi-flas...