The Thai QR code payment system has a few shortcomings, such as no options for foreigners without a Thai bank account and relying on buggy and slow banking apps. Even WeChat pay and AliPay in China can be used by foreigners, although with considerable difficulty in setting it up. But at least that system is fast - faster than taking cash, even if exact change.
The problem with being able to link a credit card to the Thai QR code payment system is that you could pay someone in cash using credit. Banks don’t want that because it creates a system where you would be paying the bank back with its own money. There’s also the issue of who would pay the transaction fees.
From the Bangkok Bank app there seems to be something called prompt pay international, and you can possibly pay from a Singaporean account using one of their banking apps.
So what it will take is for foreign banks to join the prompt pay international system. That seems unlikely.
So absent some fintech solution that is foreigner friendly, it’s going to get rocky for people without Thai bank accounts.