Yes, you need to include contracts of employment. Who gives you a contract? they can write a letter saying they are ok for you to work remotely? Portfolio incs resume, letters of recommendation/appreciation from clients. website info, sample of work etc.
provide outline of work, portfolio, letter from employer detailing permission to work remotely in Thailand. Highlight payments from work on bank statements Also check thsi site for lots of pointers
I have read posts by Preply Tutors who applied for the DTV. Yes, you have a viable route, other online tutors have gained a DTV. Great point of highlighting you can't teach Thai students. Why don't you apply in the US within the next 3 months?