The best way is to write a custom form action that will send the Form data to an OSGi service. See:
Once you get the data to a Java service - you can invoke a third party REST Service using HTTP Java APIs.
This would be the best way to address your requirements.
I already went through those links.
But my question is slightly different.
In case i DO need to send something directly to an End Point, Do i still need to go via AEM?
I am thinking why to increase the LOAD on AEM when it does not need to process a request.
Sending data to a 3rd party endpoint is a custom operation. The logic needs to be somewhere.
In the case of a Form - when you do a submit operation - you need to submit the data where it can be read and then sent to a 3rd party endpoint.
To address this - Java is the best way. AEM is build to handle this type of operations. I would recommend using a Sling Servlet for this use case