Another facet to look at is, if you start down the agent road, you're likely to use them every year and they cost a lot. I'm here on a marriage extension, but if it were a retirement one with an agent, I'd be in for ฿400,000.00, nearly 10 grand stirling.
IF you have the required ฿800,000 then just follow the procedures to get a non-o visa, and then a twelve month extension. There are certain conditions with regard to the funds you bank.
Once you have the twelve month extension, you need to report your address every ninety days, the first time in person, thereafter on-line.
It's pretty simple.
If you don't have the funds, then you'll have to use an agent, who will magic a bank letter to show you have 800k in the bank.