I'd choose FS Chiang Mai over the MO which many on this forum have described as "Disney-like" (and I concur).

I think just spending two nights at Trisara (and one of those a late arrival) is a waste of a beach vacation. If you're going to visit Amanpulo at the end, I'd eliminate Phuket from this itinerary.

I wouldn't miss out on Bangkok - it's a fascinating city, and a good one-day guided tour will introduce you to some incredible sites (such as the Imperial Palace and the Reclining Buddha).