Did you recently work? If you did you could provide a tax return, payslips and a letter explaining you quit your job and now taking a break to travel.

Last resort i think you could have your dad write a sponsor letter that he will cover you financially while you live in Thailand, should your 500k savings run out. You'll probably have to include his bank statement too. Not sure if it'll work, they are getting so much more critical with soft power

Last month my friend who applied for soft power in hcmc was asked for proof of income, even though his application said he was unemployed. He had to submit tax return, 6 months of pay slips and wrote a letter he quit his job and is now taking a career break