I had done the application about 3 weeks before the visa extension (marriage) expired and the extension was valid for 4 weeks after it expired so it took them 7 weeks to process the renewal.
My last renewal i went the day before because the actual date shown was a holiday. Got told to go back the following week amd not worry about it being after the day stamped.
Fill in the arrival and just the basics on the departure. If you were to change to stay your extension of stay to say a retirement option then you could be using the same TM6 in 20 years time if you don't leave Thailand.
Check that you have all the documents you need for the marriage certificate. I seem to recall a letter is require from the embassy saying you are free to marry and an officially translated passport are required.