Should my uncle extend his tourist visa before applying for a retirement visa in Thailand?

As I understand it there's no need for money to be seasoned when applying for the initial non-o visa in country but it does have to be transferred in from overseas. The extension would be applied for before the initial 90 days expires and the money needs seasoned 2 months for the extension. So the initial transfer can cover both as long as he doesn't dip in to the 800k

At least he needs remaining days no less than 15days (or 21days at some immigration office such as CM) with his stamp to apply for a new Non-O based on retirement.

Depends on how to meet the financial requirements, he may use money in a Thailand bank (you may or may not need to show proof of that fund is transferred internationally) or an embassy letter/affidavit for funds in your home country.

He was stamped in for 60 days on arrival. He has a 30 day extension available to him.

Have him go to immigration office and pick up list of requirements to get the Non O visa for retirement.

Following it being received apply for one year extension of stay as retiree.

His 800,000 Thai baht needs to be in Thai bank account in his name two full months before he applies for the one year extension.

Which immigration office will he be using ?

Does he have the required seasoned funds? If so, go for the retirement visa.