What documents and process do I need for a retirement visa in Thailand, and will 60 days be enough time?

If you already have a Bangkok Bank account then your plan should be okay.

Expect to be confronted by your bank and forced to sign a document that gives them permission to freeze your 800,000 baht for 4 months per their new policy. Anyone trying to get proof of funds for immigration on money that has not already been in their account for 4 months must sign this document to freeze the money for 4 months from the day the letter is issued.

Suggest showing them what you think of their policy by trying to open an account at a different bank and using the new bank for your 800,000.