Want to share some input about my Bangkok based broker: https://www.sbito.co.th
Remarkable commission as low as 0,075%. Very friendly staff, all communication in English, prompt replies. No pestering by phone calls, messages, or recommendations to lure me into specific (obsure) securities. Sbito is part of the Japan-based SBI Group.
As for the Thai stock market my impression is it is for gamblers, but if you pick your shares well you might find some real pearls, like TISCO which was mentioned earlier. Take a look at their payouts and dividend policy. It is strong for the last ten years. At present 7,85%.
The share price does not fluctuate erratically. It remains very stable at around 90-100 baht. If you're not looking for big gains in stock price but prefer stable dividends, Tisco is the choice for you.
Do your own research...