I have heard that some immigration offices will check the source of the funds and want it to be a pension. Others don't. Check with the specific office.
This happened to me once, for one transaction, and I spent about 90 minutes in k-bank getting the 'advice' paperwork. I had it in my folder when I went to immigration. They didn't ask for it.
I'm suspecting it depends on the officer and the office. Some would accept you showing statements from the money source, and corresponding deposits in your Thai bank. Others wouldn't accept it if it were plated in gold and stamped by Moses.
I'm still waiting for an apology from anyone who said I was full of it when I said extending the DTV in country would have similar requirements to getting it in the first place. I got called a fear monger.
some agents also send your documents to another immigration office than the office you're supposed to go to. But it's the office in which they have a contact.
In future years, an immigration officer may wonder why you've been living in one area and getting your extension in another.
A DTV does not require health insurance. I'd suggest you read up on extending the visa in-county vs heading out of the country and re-entering, well before your first 180 days are up. Now, would be a good time.
Phuket is very easy. They have a drive through window. It takes one minute. You hand them your passport through the window and they hand it back with your new report stapled in your passport.