I'm doing the same trip in April, I will update you. Here's my iti:
US-HK-BKK-KTM (March 15)
Kathmandu - Bkk (3 days)
BKK - ChangMai - (4 Days)
Changmai - Phuket/Koh Phi Phi (4 days)
I'm flying via Thai Airways.......very cheap, book thru their website.
ChangMai - Phuket, I'm going via BKK, direct flight is very very expensive.
Book your hotels thru Agoda.com.