Kowloon to Airport - somewhat tight timeframe

I am taking train G79 into Hong Kong from Chinese Mainland arriving at approximately 18:00. I have a flight at 1AM so I’d have time to go through customs for up to 3 hours then ideally catch the AirPort Express immediately. Is this possible/smart or should I take a much earlier train (not many options) to have “peace of mind”? I wonder if the trains are ever vastly delayed or if somehow customs might take more than 3.5 hours?