One or the other. Easier to have 800,000 BHT in your Thai bank account. 2 months before and 3 months after. Minimum balance 400,000 BHT. More work the other way but possible
Go to Immigration office yourself. This way you know for sure it is done. Otherwise go to a Visa company but I am sure you will pay more. You do not want a fine or face possible deportation.
What I wrote was if you did not have an income. I was mistaken with the months prior to and after. It is 2 months before and 3 months after. Go to immigration now and ask them.
You are right you need to keep a minimum balance of 400,000 BHT at all times. Three months before you renew the balance has to be 800,000 BHT. After two months the balance can go down to 400,000 BHT again. You will need photocopies of your bank book now going back 12 months. Don’t forget Bank letter and your bank book updated on the day you go to immigration.