Nobody asked yet, but what is your overall budget for this trip?
Overall I think you have a good plan. As some have said, it's not the most relaxing. You will see a good variety of things and it should be a good time if you're an easy going person.
I would probably skip Angkor Wat on this trip, or skip out on something in Thailand. You need a solid 2 days for Angkor Wat and it will be very hot at that time. It will not be relaxing at all. Just to give you something to think about....fly into Siem Reap in the afternoon. That night buy a 3 day Angkor pass and go to a temple and see the sunset. Next day is Angkor area all day. Third day go out for a few hours in the morning to visit more of the area. Then afternoon flight out of Cambodia. Don't forget you will need a full page visa for Cambodia.
BTW, Samui, Phuket, Phi-Phi and Angkor will be full of tourists.