I don't have experience with summer weather in Phuket. I'll let someone else comment on that.
But I have bought MANY pairs of eyeglasses in Thailand. Quality is great, prices range from good to incredible. All the brand names that are on sale will be copies. Difficult to spot, and still quite durable, I'd avoid them over the cheaper non brandname copies. Thailand sells good inexpensive frames, and they make great lenses at great prices.
The soft titanium frames are my favorite and no-name brands are available for about 200THB. I've bought clear and tinted/sunglass pescriptions for as little as 1900THB for lenses. The transition lenses that change from clear to sunglasses have a hefty price premium though at about 3000THB+ Phuket prices will be more than Bangkok, but bargaining will get you good discounts.
Be sure to bring a current perscription from home.