My PT online income was a bit less than that. I also emailed them telling the income and they said it was fine to apply as did London, Vietnam, Manilla, no minimum income etc. When I applied, I got the miserable lady others complained about. She kept saying " not enough money" even though I showed her the email. She was fine with adding investment income.
The funds for the DTV need to be in a personal instant accessible account not a business account. Vietnam is very popular to apply for the DTV, may as well apply there as you live there.
Fukuoka and Osaka are much friendlier than Tokyo. You can find several posts about the difficulty of applying in Japan, mainly Tokyo. Make sure you have 6 month plus course and demonstrate your need for a 5 year visa. Japan often issues 6mth visas if they feel you don't need a 5 yr visa.
Yeah, it should be like that across the board but a 20kTHB salary and just the 500k has more chance of rejection in certain embassies. A friend in Tokyo was rejected with low income and just the 500k. He reapplied with substantial savings and approved. I added my investments to my application in Taipei as they would not accept my new full-time contract. I was approved on PT low online income at that time; doubt I would have been approved with just the 500k.