What documents do I need to extend my E-VISA Non-Immigrant O for an additional 30 days in Chiang Mai, or can I apply online?

No. Unfortunately, no 30-day extension is available for Non-O entry. You have to apply for a 1-year extension or leave Thailand.

Or if you have a legal Thai family member, you may be able to apply for a 60-day family visit extension. It depends on the reason for your initial visa and your local immigration office whether you can use it or not. Some offices accept a 60-day family visit extension only when the reason for your initial visa is related to the Thai family (marriage or raising a Thai child). Some allow you to apply for it on most types of entry.