What are the next steps after being approved for a Non-O retirement visa in Thailand?

Sounds like you are over thinking this .Brendan’s advise is good to follow

If you don't already have a Thai bank account, you need to get that quickly, within a few weeks of arriving.

Then you transfer 800,000 baht into your Thai bank account. Once the funds have been in your Thai bank account for 2 full calendar months, you can then go to immigration and apply for the 1-year extension.

You should go to immigration as soon as you get the chance and ask them for their handout of requirements for your specific office. Each office has their own requirements, so you want the exact requirements from your office so you can prepare everything you need once your money is ready.