I absolutely love Rayavadee, so that would be my recommendation in Krabi. I would only choose Phuket if Trisara were within budget, as they have by far the best standard accommodation.
Re Chiang Mai, if push comes to shove, I would actually consider less expensive options if that would allow a higher budget for Golden Triangle and other parts of the trip. I think a suite at Anantara Golden Triangle provides excellent value, if FS is over budget. Anantara Chiang Mai is also a great choice.