I’d go for it. One of your last chances before the boy starts school. ABC beaches will be awesome; same with Seven Mile Beach in Grand Cayman. I cruise for beach days, don’t even bother fighting the crowds in the pools onboard. Cozumel is loaded with beach clubs, reasonably priced all-inclusive or pay-as-you-go.
B2B is easier if you have the same cabin. We got a fruit basket, champagne and a group picture. I actually jumped in a Uber and went to buy extra wine and soda. Had a “In Transit” pass to get back onboard, skip the line.
9/16 is still the tail end of hurricane season. I’d be looking at the first 2 weeks of Dec. on Celebration just to have 2 weeks to try all the food venues. Prices are great. You’ll be fine on Horizon and the ABC itinerary is the bomb diggity.