Yes it is the law ππ But never been asked all the times except one time... and I was the only one in the queue, no Malaysian people were asked to show the 20,000 ππ I asked to talk to the immigration chief and in his office he had a big old poster where it was written "20,000..."
Also, now with Siambank you can use only one phone.
Better to have also an account in Bangkok Bank. No need to get the sim to change a phone. Just need to know the number of your atm card and your code
I am not sure that roaming is better because I don't know if they need the simcard only or the thai simcard connected to a thai network. You can try it. Not very expensive.