this is incorrect, if you have a visa then you would open the bank account in the first 30 days and put in the money. Then you apply for an extension of your visa, not for a new visa. You have to have the money in the bank account for 60 days.....your visa would be for 90 days so that is why you have to open the bank account within 30 days.
Just beware that advice/information about Thailand's regulations and procedures given now for an event that is happening in 3 years may not be particularly relevant when you are ready to start the process. I understand the desire to feel comfortable about something, but if possible just relax and as you come up to that 6 month mark you can check/ask and get feedback that will reflect better the reality you will be dealing with.