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.
You need a stamp like this, it can be a single entry or multiple entry, check your passport or buy one at local Immigration OR just before departure on airport
YES, you can apply for the 1 year Extension of Stay based on employment in Thailand.
BUT you need to meet some simple requirements, like salary payments to staff, paying income tax and a few more, visit your Immigration Office to get the list of requirements, tell your staff to start up the copy machine, tick the list when copy is made and when all done, go back to apply.