Ok..In Japan foreign residents have to supply income, local and city tax records to extend their visa. Definitely do not see it being asked for at the airport as the OP mentioned.
I have seen one from London, one from USA who were on METVs and got the DTV. On arrival at the airport, one mentioned being asked about the 2 visas and the IO cancelled the METV giving entry on the DTV. Also some on ED visas awarded the DTV before it was cancelled. These posts from last year. Not seen such posts this year.
Anonymous participant You don't have to show income but they can/often request evidence of income via a tax return, payslips or bank statements with income deposits etc.
Another Tokyo applicant was also issued 6 months visa while applying for a DTV. Tokyo offers a pre screening appointment to determine if you are suitable for the DTV. Futire applicants should make use of this service before applying online. You can apply again elsewhere. Other applicants have failed at one embassy then successful at the next. Try and contact them to see if they can give a reason why they gave you 6 months.
Yes the 500K must be in a personal account. Detail how you run the business online. Write a letter to yourself giving permission to work remotely in Thailand. Seems strange I know but it is needed. You can find other business owner posts in the threads to get a lot of useful info.