I have done that trip to Samui so many times from the UK I know how exhausting it can be. I could not imagine another overnight on the top. I am older than you though. Don't forget that if you want to get the most out of the full moon you need to be in good feckle before it starts.
You may not be aware that if you are staying on phangnan for the full moon most places wont take bookings for less than 3 nights at that time.
Samui is a good place to stay for the night.

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Kiss
God laughs at people who make plans.