I can't speak for Chiang Mai but you are likely to need 12 months bank statements showing the DWP payment going into a Thai bank account.
You can take the account down to 400k (if you wished) until 2 months before you apply for your next extension.
Just to mention that you may first need to show immigration that you have complied with the terms of the previous approval (ie 800k in the bank etc). You must also evidence the 50k pm of income for 12 months........................a sort of double whammy.
Drive to Don Muaeng, fly and then rent a car at Buriram airport.
Avoid soi minibus (the visa run road to the Cambodian border) it is full of road works.
Take the Chachoengsao road through Kabin Buri then the 24/218 if you require Muang Buriram.