If you enter visa-exempt, you could apply for a Non-O visa from Immigration and then apply for an annual extension of stay for retirement. Assuming you meet all the requirements which are mostly financial. I don’t understand why people are saying you would have to leave Thailand.
Sorry to be so skeptical but has anyone actually flown on a Bangkok Airways "sealed flight" from Bangkok to Phuket. That is, landed in Bangkok, then arrived as an international arrival in Phuket (not in Bangkok).
Yes, I've seen the "plan" but has anyone actually done it. I'm very skeptical because there has long been talk of such a thing and to my knowledge it's never actually happened.
As others have said a 60 day tourist visa with a 30 day extension of stay for tourism is best to start.
Depending on your nationality, you may qualify for easy retirement extensions based on embassy certification of pension of 65k baht per month. Don't even need a Thai bank account for this. Only one of you needs this other can get dependent visa. If go down this path get your Non-O visas outside Thailand (dependent following spouse visas aren't issued in Thailand. Get your Non-O visas from Thai embassy or Consulate in your home country. Americans, Brits, and Australians can't do this because their embassies unilaterally stopped providing the income letter.
Agree you don't need elite visa and don't need an agent.