Ishigaki or Zamami?

Hi all,
My husband and I (35y) will be travelling to Japan for the first time in July/August and will be staying for 3 weeks. We are still in the very beginning of our planning, we just know that we’re flying into Tokyo and leaving from Naha on August 6th.
We want to spend the last 4-5 days on one of Japan’s beautiful islands. Which one do you recommend? We want to relax, go snorkelling, hiking if possible and eat good food. It shouldn’t be too crowded and we’re not interested in nightlife.
Would you recommend Ishigaki or Zamami or maybe another island? How do these islands differ? Keep in mind that we have to be in Naha on the 6th.
Thank you!