If you can assure to be in Thailand every year in the month your extension must be renewed, then get a nonO retirement and park 800k baht in a bank.
But from what you said, I’d go for a DTV which is valid for 5 years. But with this one you aren’t allow to stay more than 6 months. So be sure to count the days correctly. ;)
I gave all documents to the agency (which btw wouldn’t start without having checked that everything is good), but they asked for more than the requirements.