Listening for Target Rendered (Web SDK / Launch)

I have an unsupported scenario in how we are managing flicker delay.  To accommodate it, I need to know the moment when Target has completed the render of any personalization. 

I've attempted to do this via actions in a Launch/Tags rule, but even using "wait to run next action" on the send / render for Target, it still fires before the render has actually occurred.  I'm assuming I need to set an event listener or similar, but don't know what to listen for.  I'm even willing to do the entire send as custom javascript with a ".then" to handle the response, but I'm not quite sure how to make it work on the actual render event.

Thanks!