Santorini Excursions Question

I am planning my first cruise experience in the fall and trying to figure out activities. There are only two cruise ships scheduled in port the day I will be in Santorini, but I am nervous about lines for the cable car and how long it will take to actually get from ship to island. I have my heart set on a private excursion in Megalochori that starts 4 hours after arrival, and I'm considering booking a Celebrity excursion in the morning so that I can get off the ship more quickly and easily. The problem is I would need to leave the Celebrity excursion early to get to the private one. Is that a jerk move? Or is there a better way to manage this?