The backend could be just...

    if( is_numeric($_GET['current']) ){
      file_put_contents( 'current.txt',$_GET['current']);
    }

Then could post it it when the clock advances by a second, shortly before the slide advances on the main screen. Aggressively poll it on the clients to figure out how many ms after the whole second to poll.

All the screens would advance simultaneously which would impress the developers.

If someone in the audience has a question they can press a button to have their face and audio streamed to the big screen. Modified by LLM of course, or it would be pointless to have.