You should have the option to "self-debark" on most or all RCI cruises now, which means you don't put your luggage out on the last night but rather cart it off yourself in the morning.
But if you need to have your luggage removed from the ship for you, I suggest you simply inform guest relations that you need to debark early. Having a late flight shouldn't require you to be stuck on the ship until late in the morning. There are many good reasons for needing to debark early, and many of them do not involve your homeward transportation schedule. For instance, I drive to cruises from the Port Of Mobile but that doesn't mean I should wait until the last call to leave the ship. That could be an hour or more of precious drive time lost. I also have family living in south Florida, and even though I had late afternoon flights from Miami, I've requested and received early debarkation so I can visit the relatives for a few hours.
Just tell them you need to leave by "whatever" time and they will accommodate you.