Okinawa/ Keramas Trip Review Request

Hey all, am looking at taking my parents to Okinawa (Age 60+) for a relaxed trip there and I've got a rough plan, but would like some feedback. This is the first time in Okinawa, but I've been Japan a few times before and also know conversational (basic) Japanese (Studied for like 5 years). Do have an international driver's license and plan to rent a car.

The Okinawa leg of the trip would be 18th of September to the 24th (night arrival/ morning departure so probably more accurate to say 17th-23rd for a total of 6 days). With 1-2 days on Okinawa around Naha, before catching a ferry to one of the islands (Tokashiki or Zamami) for 2.5 days, then another 2.5 days in Okinawa (North side around Onna or Nago).

For Onna, main things of interest would be snorkeling at the blue hole, maybe the Nakagusuku Castle Ruins. Around Nago would be the aquarium, pizza in the sky, maybe the pineapple park and Nakijin Castle (probably just one of the castles). Let me know if I've missed anything of interest!

For the islands, I was originally thinking Tokashiki island with accommodation around the Arahen beachfront, but have seen a bunch of people recommend Zamami, since it has two beaches and more restaurants/ places close later. It also seems that you need to book tours now for a lot of the snorkelling if you wanted to explore unrestricted?? Is that true for both places? They are both not great at swimming (both can breakstroke) and it seems like there aren't many lifeguards, so would be looking to rent life jackets anyway. We are from Australia if that makes a difference (in terms of beaches, I've heard ours tend to be rougher, so maybe I'm overthinking??)

Would be great to get some feedback! Have some flexibility to change things around, and even add a day (subtract from Osaka/ Kyoto) if the above is going to be too rushed. Also are typhoons a worry/ should I not be booking things around Okinawa/ the coast at all? (I can scrap this plan, I only just started reading about typhoons with the latest Shanshan one – hope y'all are safe)

Side Note: Is it too hot/humid for (coastal) hot springs at the end of September? (30th-ish) Was considering a leg to kii-katsuura down from Osaka for a few days.