That sounds really fun and I hope you have a fantastic trip!
Castaway Cay is fantastic and there are things to do other than just beach, running/walking area, bikes, etc. Unless we missed it, on RC private island it's pretty much just beach/pool (which got incredibly crowded) or you pay (and it was a lot per person--$150 for the water park, $100 for the zip line, $100 to go up in the balloon) for each other thing. I do not think when you factor it all out, they are less expensive than DCL.
I wouldn't worry about being a person who can't see past Disney--I will be the first to say when I speak of corporate greed they are ALL guilty, including Disney...BUT there is something called the Disney difference and it is for real! Service industries need to focus more on these things because with inflation and costs what they are, people have choices and expectations in travel. In 2021 we planned a vacation to Disney after masks went away. We intentionally planned and waited so we wouldn't have to wear them in the parks (not a political thing, just not comfortable in the Florida heat). They ended up reinstating them in the middle of our trip. We went to the guest services and asked to be released as it wasn't what we hoped for, and not only did they release us and refund the rest of our hotel days there, they offered us comped tickets for each and every person in our family to come back again good for 5 years. That is the Disney difference. I don't feel "entitled" I just feel like when a guest comes to you with a complaint you aren't delivering what was marketed, you do something--anything--to try and off set that negative experience. Royal did zero--and the email stating a policy and reminding me I can book another cruise with them was just an insult. We had another experience in Las Vegas one time where they put our kids room on a totally different floor than ours, the hotel was booked solid so they couldn't move us--they at least offered us food/beverage credits and tickets to a show to offset the inconvenience.