you aren't comprehending the "very first year" part of what I said. I've been doing this for 14 years, and using the income method for the last seven years.
that works great after the first year's extension, but you can not do that anymore for the very first one year extension. You do understand that right?
they can get the initial 90 day type O visa in the US by using the monthly income method, but, they can only get the one year extension inside Thailand, and as Americans can not use the monthly income method because they would need the income guarantee notarized document from the American embassy, and the American embassy no longer issues that document. As such for the first one year extension they must have at least bt800,000+ in a bank account, and being that both are over 50 years old Thailand won't issue trailing spouse visa extension anymore.
to do that, and not pay tax in your home country, would be determined by the dual tax treaty, if any, your home country has with Thailand. You don't get any benefit by paying taxes in Thailand on a DTV.
you are not working inside Thailand. You are working remotely, and Thailand trusts you to file the appropriate tax in your home country, or the country you remotely work in, on the money you bring into Thailand. You are trying to overthink this.
This will better let you understand your tax liability. In order to file taxes in Thailand you must have a TIN, taxpayer identification number. Go to your local revenue office with your DTV, and no work permit, and try to get a TIN. That will answer your question about paying income tax in Thailand. It is that simple.
they can also call you in for an interview in some cases, and if you are out of the country that can be a problem. At some point when they review your application they also check with immigration to see if you are in fact in the country.