Non O Marriage Visa question: how early can I extend my yearly before the due date? Is it like the 90 day report: 7 days before and 7 days after?

When doing a border run, and still having a valid Non O (Marriage) visa, what do say to immigration at the airport?

I need to start fresh due to a visa agent doing unscrupulous things for my last Non O and need to just come back to Thailand with the visa waiver. I am Australian so this is what I want to do to recommence my time in Thailand and apply for a Non O marriage visa myself without an agent.

So what do I say when I exit and re enter Thailand?