As others said. Get a NonO from the visa site. Retirement if you can, otherwise marriage.
For retirement you need to be 50+ and best have 800k to spare. Enter the country, open account, deposit the 800k (make sure that the bank can identify it as foreign money!), and do the extension near the end of your 90 day visa. You can do it with 65k per month but ask your immigration for that just after arriving!
Otherwise: marriage visa where you need only 400k in the bank.
If you have the money, go for elite visa or LTR, but that’s on another level.
Just don’t enter on visa exempt because you can’t open an account as a tourist.
Edit: and I did it wrong on first try, believing that it’s an average of 65k. But it wasn’t. Must be 65k each and every month. So I started twice on basis of the 65k monthly.
And don’t take the OA route. Better go the normal O for retirement.
It doesn’t need a health insurance, which you need to exchange after a year anyway into one that really fits your needs and is not from a list of expensive companies.
Plus: you don’t need an agent. The majority has done it without agency. It’s really simple. The agency just does it for you.
Except: if one doesn’t have the money, then they bribe for you and you can’t evade it in the future.