You might want to say where in Thailand you plan to move to since this will help others give their advice. The immigration details can be a little different in each of the provinces.
The O visa is a 90 day visa. You extend this visa annually. I kept the 800,000 baht in my Thai bank account year round. I found this to make the 1 year extension process, in Bangkok, to be very easy to do myself (1,900 baht fee annually).
If you are over 50 years old the typical visa used by retirees is the non immigrant O visa. As an American you must be able to deposit 800,000 (about $24,000) baht in a Thai bank account. There are more details to this but over 50 and 800k are the basic requirements. If you don’t have the 800k then you might consider using a visa agent (which was never my choice).
Other visa options are the OA visa, Elite visa, pensioner LTR visa . . . possibly the DTV visa.
Don’t even think of staying long term on a tourist entry unless you’ve never been here and are you are coming to see what Thailand is all about.
I went to look for the new office a few days after they were supposed to have moved and it was still under construction. Not open. I will check again soon . . . or maybe I missed something.
A bit of advice - ask the information desk which elevator to take to the 7th floor. They don’t all go there.