Too slow for a true rumba in my opinion, unless you are professionals and do a choreographed lyrical dance with a story to it. That's not what you are looking for, obviously. I still think slow Fox Trot with nice sway steps is the way to go on the McGraw tune. There is a slow social dance version called the Foxy in some studios. Take the song to a dance studio, tell them you want it to be your first dance at your wedding, and see if the instructor cringes, or comes up with a decent idea. If you like the idea, take a few lessons. If you don't like the idea, just figure you at least got a second opinion. Google "slow country dance" and see what comes up on the radar.
Whatever you decide, you're going to have fun.