Anonymous participant they’ve asked you for the letter. One of the conditions of the visa is that you do not earn money from Thai sources. It’s good to let them know that you understand that.
Instead of trying to apply your own personal version of logic, just write the letter.
The clinic would be who you need to speak to. Ask them if their treatment has already been used to apply for a DTV. What documents they’re able to supply to support your application.
If they haven’t supported a DTV application yet, ask them to research it and get back to you