Ok, so the solution was to build the webhook integration with node.js and ngrok. So it is not dependent on the web server.

Also, out Apache is still running with the prefork module due to some old legacy code that is not MPM-compatible. Apache prefork module is NOT adequate for higher loads. We are in the process of upgrading all the old legacy code to be MPM compatible so that we can use one of the MPM modules. That might also be a solution if you want to use PHP for the webhook

But the node.js + ngrok solution works very well.

Hope this helps other people that might have a similar situation.

AndreT