OP, note you need a Thai bank account in your name only.
Do you have one.
Also need to show funds (800k) came from abroad.
Other requirements outlined in these attached threads
https://aseannow.com/topic/1282251-visa-exempt-to-non-o-retirement-dates/
https://aseannow.com/topic/1301615-converting-visa-on-arrival-to-non-o-retirement-based-in-nan-province/
https://aseannow.com/topic/1297092-non-o-visa-retirement-purpose/