There is no 7 day exension in Thaiiland. However, if you apply for a 30 day extension and itt is denied by Immigration, then you get a stamp in your passport advising that you must leave Thailand within 7 days - but it is not an extension of any description..
I recommend that a Retirement Visa is the best for you and that you should obtain it from the Thai Embassy/consulate in your country prior to coming to Thailand. I say that because if you come to Thailand on a Visa Exempt stamp or tourist visa, then you will be unable to open a bank account in Thailand and a Thai bank is required to obtain such a visa while in Thailand.
I am 78 years old and have lived in Chiang Mai for over ten years. I chose Chiang Mai because I have a number of serious medical conditions and because the medical (GP, specialists and hospitals) support here is excellent and, in my view, affordable. As an example I can usually obtain walk in appointmets to my heart and diabetes specialists in excellent private hospitals about 1,000 thb, which is cheaper than seeing a GP in my country. The smokey season has not been a problem for my family. Last year was a good year with not much problem and the Thai Government has promised to fix the smoke pollution problem. We do however, have an air filter machine for each of our bedrooms and another for the lounge/living area.