Apply for the Non Immigrant O visa based on being over 50 years of age and for the spouse the Non Immigrant O visa as dependant at the Thai Embassy in Pretoria who are very helpful and have good service.
Get 90 days on entry and before these 90 days ends, apply both for the 1 year Extension of Stay.
What kind of stay do you have and for what kind of extension of stay did you apply or what is the reason that the Immigration Officer told you when they not approved your application.
I guess you visit every year an Immigration Office with documents of the organization and get every time a new stamp with the text " Extension of Stay permitted up to xx/xx/xx" .
You go with a letter of the volunteer organzation to the Immigration Office to cancel your stay at your last day of being a volunteer and apply directly for a new Extension of Stay based on being over 50 years of age (retirement).
Make sure you meet all the requirements for the new extension before you cancel you legal stay in Thailand.
That benefit has an opposite side as you need money to pay your bills in Thailand. So never sending money into Thailand is nothing more than a dream scenario
FYI there is a internal war going on in Myanmar, Burma changed the name several years ago and borders are closed for tourism and border runners from Thailand.
If you cancel your permitted to stay inside Thailand on day xx/xx/xx. you have to leave on that day xx/xx/xx and not 6 days later or whenever you choice.
So in your question cancel on Jan 3 means leaving Jan 3 before midnight, as the organization is willing to adapt the last day of you working for them, pich a day so you have time to pack your bags and leave after you also cancelled your work permit.