the other route you can go is private tutoring. You will still need work permit, which narrows your visa options, but pays better if you can get a few weekly gigs. Still legal and I only recommend the legal route cuz why as a guest in another country would you willingly do something to get kicked out? My thoughts anyway. Hopefully this is enough information to get you googling and learning more on your own.
you don’t need to get double certified. TEFL is recognized internationally. You might have some other requirements depending on who hires you i.e student teaching or assisting, etc., but your degree will need to be apostilized by the Thai embassy or consulate before you go.