These are the steps I took. I came in on a 30 day exemption and then applied for a non-O. I then seeded the 800k for 2 months and then converted the visa into a one year stay based on the retirement.
if you're staying here long term you have to go to the immigration office every year anyway to renew the retirement extension. And then there are 90-day reports. Last time I renewed my extension it took about 20 minutes. So whatever way you do it you can't avoid the immigration office.
On the contrary. It is extremely easy to convert from a 30-day tourist Visa exemption to a Non O and then a one-year based on retirement. And you don't have to prove the 800k came from abroad. I should know because that's what I did.
If you get a non O overseas you have to provide more information about finances including a background criminal check. If you get it in Thailand there are no such requirements. I converted my 30-day exemption then put the 800k in my account let it seed for 2 months and then I applied for the yearly extension based on retirement.