An OA is a one year multiple entry visa. When you leave and return during the first year you get stamped in for another year or up to the expiration date of your health insurance.
I assume you know that to get off the OA you must leave before the first year expires and return after it expires. So you don't want insurance . . . that is up to you. The next question you have to ask is what are the financial requirements you are willing to put up with? The O and OA are identical except for the insurance. I switched from my OA to a pensioner LTR and I am quite happy with that choice but there is a steep financial requirement. Or you can look at the DTV but regardless of it being a 5 years visa you are still basically a tourist. That may or may not fit your retirement lifestyle and yearly travel. I can't say it is a wise choice to come and go on 60 day visa exempt entries and extensions but if you are willing to live with that uncertainty then . . . again . . . up to you.
Marry her because you want to get married not because of the visa option.
There are two interviews of Naruchai Ninnad, Deputy DIrector-General of the Department of Consular Affairs at Thailand’s Ministry of Foreign Affairs. One on the YouTube channel RW4U and the other is a Bangkok Post podcast. Search for “Deeper Dive Thailand” on a podcast app.
There have been several interviews of Naruchai Ninnad, Deputy DIrector-General of the Department of Consular Affairs at Thailand’s Ministry of Foreign Affairs. In one of those interviews, I think the podcast. he specifically said that you had to return the next day. Feel free to do whatever you want.
If you want to listen to the podcast then go to your podcast app and search “Deeper Dive Thailand”. It is a podcast from the Bangkok Post.
Short answer - you can extend the non O indefinitely as long as you meet the extension requirements. But even for my 10 year pensioner LTR, when it ends then it ends and then I can apply for the same or different appropriate visa at that time. If you were to end your non O you could then apply for it again at a later time.