You can stay in Thailand 6 months a year on an initial O visa based on retirement. Just leave using a re-entry permit and return before the 1 year permission to stay expires. Cost: ฿1.900/year at immigration plus ฿1.000 for the re-entry permit. The DTV isn't actually designed for retirees.
You can get an insurance when on a O visa. No need to go for the O-A with a mandatory health insurance if you're thinking about extending the stay another year in Thailand. Check out Pacific Cross. A reliable insurance company with loads of affordable plans.
Brian Gorecki In which thai bank won't you get interest on for example an FDA? A ฿3 million baht cover insurance is not considered cheap. That's probably the reason he asked about the proposed change in minimum cover.