you can transfer money from Nationwide to Wise on your phone using the Wise app and the Nationwide app together. It transfers the money in about one second. Then it takes about another 2 seconds to transfer the money from Wise to your Thai bank account.
When I transferred my visa to my new passport recently there was no mention of a letter. All immigration wanted was the old and new passport. They have all your details on the computer system.
You are right in telling her to save her money on paying agents fees if she has the 800k. When I got my retirement extension I entered on a 30 day exemption and converted it to a Non B at the immigration office and waited for the 2 month seeding period for the 800k and then converted to a Non O and then the yearly extension. I never had to leave the country. It cost me a total of around 4,000 baht for the 2 visas. It was very straightforward and easy to do by myself. I don't understand why so many people are telling you to use an agent.
you never have to use an agent. I did all of that myself. From 30 day entry converted to Non B then Non O and one year extension all without leaving the country. 2,000 baht