If you're using monthly deposits = at least ฿65k/month and you have a valid income letter (not older than 6 months) from your embassy,you just need the TM7-form and the same documents as when you applied for the 90 days Non-immigrant O Visa based on retirement.
A 60 days tourist visa from back home and then extend it another 30 days at the local immigration office. This is by far the easiest way to get 90 days. The extension cost ฿1.900.
You have only 60 days on that METV. It's officially not extendable another 30 days at immigration. You can apply for the 90 days Non-immigrant O visa based on retirement after 30 days,but you also need ฿800k in a thai bank account. This will probably be your most important issue.