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.