Re multi hotel stays - Four Seasons may negotiate discounted upgrades. There is also the possibility of the free two nights at other FS if booking three plus nights at FS Golden Triangle, if that offer is still available. That could take care of two nights at FS Chiang Mai or Koh Samui.

Minor FS are of course managed separately from Anantara, so multi hotel discount is less likely across different brands. However, wholly within FS or Anantara, something may be possible. There is no published offer for multi hotel discounts that I am aware of, but I think a few emails to management of the relevant properties might yield results. They would definitely want to win the business.

An all Anantara or all FS itinerary would very likely make upgrades easier to negotiate. (Anantara Siam Bangkok may also negotiate in conjunction with other FS stays, as it was previously FS BK.)

If Anantara - join their membership program (which is GHA membership) to avail of discounted rates and benefits.

Anantara has its Layan property next door to Trisara, which would likely be within budget.

This is true, but that is of no help to someone wanting free accommodation in order to stretch budget on other parts of a larger itinerary. Point valuation goes out the window if one is not trying to maximize redemption value, but simply save cash.