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

The extension visa is actually the same process as to the non o that you applied except its in Thailand. The first thing you do when arrives to Thailand is to open a bank account and deposit that 800kbaht. This is to replace your financial proofs when initially applied the non o

Don't believe anything onFB

Once you get here, get a Thai phone number/sim registered to your name and passport. Also ensure your landlord files a TM30 where you live, you can then request a residence certificate from immigration to facilitate opening a bank account. Suggest you don't open one at Bangkok Bank.

My advise is go to your nearest immigration office and get the information off them , go there asap after you arrive

I’m not going to give advice but please don’t worry, it’s much easier than expected, no need for an agent.

Contact Thai Visa Centre

Is there any reason why you are getting a retirement visa and not an extension of stay based on marriage? It’s only 400k in the bank then.

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.