Generally, it is often considered easier for Thai citizens to obtain a UK visa compared to a US visa. The UK offers a range of visa options, and the application process can be more straightforward for certain categories, like tourist visas. The US visa process tends to be more stringent, with more extensive documentation and interviews required. However, the ease of obtaining a visa can vary based on individual circumstances, such as travel history and financial stability.
I think it would depend on the individual policy. Many travel insurance policies have a maximum number of days out of your home country. If you are planning to live in Thailand for several months, you should probably look for insurance coverage for residing in Thailand and not travel insurance. But again, it depends on your specific policy.
Before I block you, I would like to point out that people have tried to offer their assistance and advice. The fact that a METV must be applied in your home country would be very valuable since you wrote that you are in Vietnam. Not once have you said thank you for their contribution; instead you have been snarky and rude. Your negative karma will be justly rewarded.
Not to be cheeky but finding a year round cool city in Thailand is as futile a search as a year round sunny city in the UK or a city in the U.S. without handguns or a city in Australia without drunkards on the weekend. It just doesn't exist.
I am not familiar with the BOI procedures. Maybe if your company could give you guidance. I think it would be part of the company's responsibility to assist with bringing employee's families to Thailand.