(A bit OT, but) Just thought I'd say Hi as it sounds like you have a very similar set up to me. Also in Thailand, also got a kid at an international school.
I'm Norwegian and grew up bilingual in the UK and I see my daughter doing many of the things that I did as a kid. Effortlessly switching language mid-sentence for example. I do expect her to (and hope she does) end up with stronger English than Thai. I think that'll be far more useful to her in life.
I did not grow up bilingual so I was pretty impressed when I saw my kid effortlessly switching between English and Thai. I would tell him something in English and he would immediately turned to his mother to tell her what I had said in Thai. As early as 3 years old no less. I guess that is just normal!
The question of which language is going to serve kids better in the future seems to depend on where they are going to live. I have friends with teenage kids who go to international schools in Thailand and their ability to read and write Thai is pretty weak. How are they going to be successful living here if they can barely read the newspaper or fill out a job application proficiently?
Do they have better Thai language skills than you do? I speak and read hardly any Thai, but get on pretty well.
There is a huge amount of business conducted in English in Thailand. I expect that the better jobs (in terms of prospects and pay) will continue to be English speaking ones.
In the cases I know of, including my own, one parent is a native Thai speaker, the other native English, kids went to international school. Instruction in the schools is in English, with a few classes per week of Thai language studies. The kids all speak Thai as natives but are weak in reading and writing skills. In my case, I speak Thai quite well and can read and write at a mid-primary level, better than my kid can. It's kind of funny that we think we will need to send him to tutoring to bring his Thai language skills up to par.
Yes, the international businesses all use English and the better jobs require English. But you still need to be proficient in Thai if you want to do business here, or even just be self sufficient in every day affairs.
Given all that, being an employee in Thailand, even with a good education and strong bilingual skills, doesn't really pay well. The only people in Thailand who make money are the owners of businesses, not employees. There's really nothing like the highly paid high tech jobs of which there are many in the west. Unless that changes in the future I will encourage my kid to go work somewhere else if he doesn't have a business of his own.
I'm Norwegian and grew up bilingual in the UK and I see my daughter doing many of the things that I did as a kid. Effortlessly switching language mid-sentence for example. I do expect her to (and hope she does) end up with stronger English than Thai. I think that'll be far more useful to her in life.