Sounds pretty strange. You should be a tax resident somewhere.
I don’t understand how your line of work can’t be well represented because you only stay 3 months at a time in each country? Very curious what line of work.
Depends where the money is coming from. If you’ve been getting paid in Thailand, from Thailand it would be red flags. But , I used a Thai bank account all legit and it was no issue, but they did ask follow up questions to make sure I wasn’t working illegally.
Yes you can, but. When you first get a Thai driving license it will be a temporary one, which you cannot use outside of Thailand or get an international driving permit with .
You can buy and register a bike in your name no problem.
You can go to Laos and Cambodia and Malaysia on a bike registered in your name but, you won’t be able to go into Vietnam alone, you can only take it as part of a tour with a guide.