There’s no such thing as offseason in Singapore. I had been there in December during Christmas time ( check out my blog about it) and it was great but crowded. I think Thailand would be the same
Thailand-:
Although the climate varies throughout Thailand, you can visit all year round. The best time to travel is during the cool and dry season between November and early April. In the south, the climate differs between the eastern and western coasts.
Singapore-:
Overall, the best time to go to Singapore is between February and April. Although Singapore is—for the most part—a year-round destination, the months of February to April fall within Singapore's dry season and experience (a least a tad) less rainfall than other times of the year.