It’s a lot cheaper if you get the initial 90 none O visa before you come. Also depends if you want them to front you the 800,000. My friend has just had a price of 28,000 baht, for a multi entry visa.
I did not need to show my TDAC today when I came in on a none O visa. The IO said that it was linked to your visa, if you put the visa number on the TDAC form. I was questioned about what country GBR was thought.
For a second year extension, it just has to be an international transfer of a minimum of 65,000. If you want to use the 65,000 for your first extension, the rules are different, and has to be verified income.
Get a none O solely for the purpose of opening the bank account. Then when you return home you can start to transfer in the 800,000. Once you have the money in the account, on your return, you can apply for a None O in Thailand, then get your one year extension when the funds have been in the account over 2 months. This is what I am doing later this month, opening a bank account, ready to return to retire in December.