I guess the only thing I would add is that you are going halfway around the world so don't discount the effects of jetlag. And while the heat and humidity is on par with that time of the year in much of the USA, you may need to get used to being out in the weather more so plan on doing less in a day that you would in more temperate climates.

Myanmar is changing but Bagan is rather protected since they threw all of the locals out several years ago. Definitely agree that 2 weeks in Myanmar alone is a good idea for a future trip. Hopefully the infrastructure will see improvements by the time you get a chance to visit. I enjoyed my time there, but the logistics and travel hassles were not an ideal part of the trip.

Take your time and soak in Siem Reap. Plenty to do there for several days. Make arrangements to get to some of the less busy sites that are located a ways outside of town.

As far as I am concerned a beach is a beach. There is so much more interesting in the region to see and do. But it's your vacation!