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?
if you extend at immigration you have to show continued activity so a 6 month course would satisfy that requirement. Much easier to just leave for a short trip nearby I think.
There is no minimum income requirement. Essential to have 500k THB in a personal bank account. Should show webpages as evidence of operating a legit business.The profitability of the business is not going to go against you.