It's not a requirement to proof you have income. It is however a requirement to proof you are working online. Just signing up to preply without a working history may not be accepted I think. You could try adding a cover letter to explain your situation. Good luck.
Anonymous participant because you don't need a 5 year visa if you can only work in thailand for 30 days per year. People have been rejected over this before so definitely don't mention it anywhere 😉
Its not a joke lol. You either extend at immigration and have to provide all documents again or you leave the country and come back (border run). Those are your options. Good luck.