although I now have a Thai driving license, I had been driving on my British license in Thailand for about 10 years when I had an accident. Someone drove into the side of my car and totaled two doors which had to be replaced along with expensive body work. I showed the insurance company my British license and they didn't bat an eyelid and paid for every single baht of the damage which was around 50,000 baht.
it's not an extension, it's a new Visa. A few days before your non-visa 30-day exemption expires go to immigration and apply for a non-o visa which is valid for 3 months. Tell them you want to convert your 30 day entry to a non-o visa and then convert that after 2 months which is how long you will have to have the money in the account for a minimum to a one-year retirement extension.
you are not getting an extra 2 months. The 30 days is finished and you get a new visa which you will have to pay for and then you have to wait until it's been running for 2 months to seed the money in the account before you can apply for the one year extension for retirement.
You could do what I did which is to come in without a Visa on a 30 day entry. Then go to immigration and convert that to a Non O Visa a few days before the 30-day expires, seed 800k that you will need to deposit in a bank account for 2 months and then go back to immigration after 2 months and get a one-year extension based on retirement. Then renew every year. You don't even have to leave the country.
There was a time about 15 years ago when I used to get on a plane from Phuket to Singapore in the morning, have some lunch, do a bit of shopping in Changi airport and get the flight back to Phuket in the late afternoon for another 30-day entry. Rinse and repeat one month later. Those were the days.