July and August are a great time to go to Koh Samui but not ideal for Phuket as the 2 areas have different, very distinct monsoon seasons.
For the overall spread of what you want, I would really only bother with 1 beach area and include some time in Chang Mai and Bangkok for a cultural fix.
Beach huts are getting harder to find on beaches that are closer to infrastructure and a good range of nightlife but there are still some available on Samui, particularly on Lamai beach, which would be my recommendation based on your criteria. There is a very nice casual atmosphere there and even though the nightlife is not as frenetic as Chaweng, I would see it as adequate.
If you decide to wait until November, then the weather turns around and you have a better chance of having dry weather on the Andaman Sea side which takes in Phuket, Krabi and Phi-Phi. Beach Huts are rare on Phuket but they can still be found on Phi-Phi at some of the less developed beaches. These areas have less nightlife than the central Tonsai beach but a short boat trip will get you to all the action.
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