not complicated... just have your new property owner file the TM30 with local immigration office. Get a copy of residency certificate for stapling in your passport.
you assessed the situation correctly. Guess I will have to take the next step which is to somehow apply inside Thailand for an extension of stay based upon retirement, with the 800k in a Thai bank.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.