not so easy regarding Cambodiabno border open. You can be stopped at the border in Laos and questioned and asked to provide reason for re-entry, finances etc.
the amount initially deposited can be the minimum the bank requires. The 800,000 needs to be there from the date the immigration office requires. Not the same thing.
o visa based on retirement would be the visa you need. It can be done in the US as an OA visa but requirements are different. Check the Thai embassy page.