You are right regarding the lack of 4-5 star resorts on these Islands. There is a nice Resort on Koh Kradan, not really 5 start, but they have comfortable villa's. It's called Mali Kradan.

By the way, Koh Ngai is still part of the Krabi province. I was there last year and was surprised about that myself as well. I always thought all the Islands would belong to Trang :-)