Decided to go diving at Ko Tao and to the full moon party on Ko Phangnan after all after talking with a few more friends. Here's the new itinerary.

Nov 18 Arrive in Bangkok 2am, leave for Phuket 7:55am

Nov 18-Nov 23 Phuket/Ko Phi Phi (staying most of the time on Phi Phi)
Nov 23-Nov 26 Ko Tao/Ko Phangnan (staying on Ko Tao all 3 nights)
Nov 26-Nov 30 Bangkok
Nov 30-Dec 4 Hanoi
Dec 4-Dec 7 Siem Reap
Dec 7-Dec 9 Hong Kong