Am I correctly planning for my retirement visa in Thailand?

A correction to your plan you can't get a Non-Immigrant Type O-A visa INSIDE thailand, you get that from the consulate where your passport is from or where you hold legal residence BEFORE you come here

Other than that your plan is good. Come in on a 30 day visa exempt entry, and then get RIGHT AFTER applying for a 90 day Non-Immigrant Type O <-(note there is no A) visa based on being over 50 (retirement). The reason I say get right on it is most immigration offices require at least 15 days left on your current stamp to process Non-Immigrant visa applications because they go under consideration for 14 days before you go back and get it inked into your passport. It costs 2000baht

Once you get the Non-O visa and 90 day stamp you wait until you have 30 days or less left on that stamp and go back and apply for the yearly extension of stay buy meeting the financial requirements. If you are using banked money the 800K baht (or the equivalent) needs to be in a thai bank account in your name only for 60 days before you apply for your yearly extension. Those extensions are issued on the spot and they cost 1900baht

why not cut a few steps out of the process and just get a 90 day single entry Non-O visa from the thai consulate in your country BEFORE you come here?

OR

Instead of doing the 90 day Non-O to yearly extension here in thailand apply for a year-long, multi-entry Non-Immigrant Type O-A visa from the thai consulate in your country before you come here.

That visa does require a police back ground check and a medical certificate BUT you can meet the financial requirements using money in your bank account in your country AND that visa is good for unlimited entries for the validity of the visa itself (a year from the date it's issued) and every entry gets you stamped in for a completely NEW year.

you don't need a yellow book to get an extension based on retirement. They are not connected at all.