I have a similar itinerary in March, and have been on the same years ago. Your plan is on the money. Cozumel is most developed and best for wandering around on your own. Roatan and Costa Maya don't offer a lot. I think the sloth tours and fairly short, combine that with your beach day as the beach options are much better in Roatan than in Costa Maya. In Costa Maya, there's the port area and then some offerings at Mahahual, maybe Lost Kingdom, but I'm planning on spending more time on the ship before and after checking out the remodeled/expanded port area.
I do see that the Costa Maya port area just installed a flow rider attraction, anybody there recently have any info on pricing?