it depends on your own passport and whether you need a visa or not for that country. You could go to Laos, Cambodia (only by plane), Malasia, Myanmar or even to Singapore, Korea, etc.
No. Not at all. I don't have a Thai bank account and have been using PromtPay for over a year. I haven't tried with the TrueMoney app yet (as I opened it like one month ago), but I use the native app from my home country bank to pay the Prompt Pay and it works well 🤷🏻