You are doing yourself a great disservice by not flying to the southern islands of Thailand, quick and easy via BKK. If need be, I'd happily skip Chaing Mai for Krabi if this is your first trip.

Railay beach is amongst the world's very best, and you'd be hard pressed to beat a resort like Rayavadee. If you want peace and quiet Pimalai is a close second.

In either case make sure you have at least three days and book all the Island excursions the resorts have to offer.

You're trying to pack too many destinations in too short a time IMO. Thailand alone warrants ten days at an absolute minimum, Vietnam and Cambodia are different but inferior by most measures and not worth taking time away from Thailand unless you've been there before and want to explore new territory.

(FYI I take 2-3 vacations in SE Asia from the US a year)