But the OP is talking about touring from June to September, when it would be hot in northern Thailand. I've been in northern Thailand twice in January/February, and the weather then is almost ideal for touring. Warm during the afternoon, cool but comfortable in the morning. When I was in the mountains of northern Laos in January, the weather was similar. However, I later met a cyclist who had been in the same region a couple of weeks before me, and she experienced much cooler temperatures.
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As for Sri Lanka, I was there in January/February, and while it was hot, the daytime maximum I had there was probably 31-34C, except in the mountains where it was cooler and more comfortable. Very humid everywhere, however.

Tropical nations north of the equator are generally going to have better cycling weather from December-February than from June-August.