Sure, $15 for the TRANSFER, but you're going to be completely hosed on the exchange rate. And you won't get to know how much baht you're actually going to end up with until the transfer is complete and it has been exchanged, so it will be a mystery.
With Wise you know EVERYTHING up front, what they charge and what the exchange rate will be.
That would be the most useless trip ever, because each immigration office has their own policies. So what one office tells you have absolutely zero relevance in another one.
That person lied to you. The government hasn't changed anything. The government hasn't even been functioning for the last month. That bank branch just changed their policy and don't want to deal with foreigner paperwork anymore. The one down the street has different rules.
As I said, you need to ask your immigration office before you start planning on using monthly transfers, because some don't let you use that for marriage extensions.
You cannot go to the nearest immigration office. You go to the immigration office that serves the province where you are registered as staying. That is not always the nearest office, as the nearest office may be in a different province.
If the 800k has not been in your Bangkok Bank account for 2 months at the time you apply for your extension, you cannot use that for an extension. You will also be required to keep it at 800k for 3 months after your extension and it can never drop below 400k until next year.
But none of that matters. Just go to Kasikorn Bank and get a credit advice showing that the money sent from Wise was an international transfer, and submit that with your bank statement showing monthly transfers.