Minimizing currency costs for brokers

As a Thai (tax) residents, I'm trying to find the best brokers to minimise currency conversion costs. I'm interested in brokers with large numbers of sophisticated ETFs, such as specialised ones, sentiment indicators, inverse etc, who can be accessed by Thai residents which deal with USA ETFs. I do not want the brokerage to be located in the USA; Singapore is preferred.

Many of the brokers insist on the funds arriving to their account from a Thai account in baht from where it is then converted back into the original or some third currency. This is presumably a money laundering issue. It is also a waste of money.

This very specific and precise information is impossible to uncover until completely registered on each brokerage (i.e. they don't advertise how they deal with this issue). 

I have registered with quite a few and found a good option is capital.com. But they don't have all the ETFs I'd like. I want more options. I definitely know Xtb and Ibkr require the money to come from a Thai bank. A shame because they have a good selection.

Others were unsure and it seemed their support staff could not grasp the issue; I would only find out when attempting to put money in. This would be a costly test to perform as once the money is in their accounts they start charging for inactivity etc.

Anyone who has this magical info. please let me know. Otherwise I face the tedious task of registering with each and every brokerage.