Arrive on a 90 days Non-immigrant O Visa based on retirement. Then during the first 30 days, open a thai bank account, not a joint account, and transfer ฿800k per person. Then after 60 days into those 90, apply for a 1 year extension based on retirement. Those ฿800k must be in the account at least 60 days prior to the day you apply for the 1 year extension.This is non-negotiable at immigration.