How does the 90-day reporting requirement work for a non-O visa based on retirement in Thailand?

You can do 15 days prior, or 7 days after is what IO told me many years ago.

Don't test them get in within the week prior to be safe.

You're retired, participate in the system, you can't fight it.

Just ask the officer when you get your extension stamp.

Your first 90 day report will be 90 days after your 1 year extension. When you pickup your 1 year extension from immigration they should have a piece of paper stapled into your passport that gives your first due date.

Until you're on the 1 year extension the 90 day report doesn't apply for you because you're not on a stay that can exceed 90 days yet. Any stay of 90 days or less generally isn't required to do 90 day report even if the total time combine exceeds 90 days.