@Jack ********
The additional year, being “stamped in” isn’t actually an “extension of stay”.
It’s simply that, the Visa enables you to be stamped in for 1 year, every time that you enter Thailand, while the Visa is valid. You need to have health insurance, to cover that time. Otherwise, you will only be stamped in until the expiry date of the insurance.
This enables you to get almost 2 years stay in Thailand, from the one Visa.
I think that, if you want to stay longer, it’s best to start again, with an O-A Visa, which can only be obtained from your home country.
Alternatively, you could get a Non-O Visa, from inside Thailand, or any other country.