I assume have a visa exempt entry not a visa on arrival you applied for when entering the country and paying 2000 baht for it.
1. You can apply for a non-o visa at immigration up to the last 15 days from your 45 day visa exempt entry or the 30 day extension of it. There is no minimum number of days to do it.
2. Yes if your are living in Bangkok.
3. No insurance it required to apply for a non-o visa or a extension of stay based upon retirment.
4. Requirments for the non-o visa application is here: https://www.immigration.go.th/wp-content/uploads/2022/02/9.FOR-RETIREMENT-PURPOSES-50-YEARS-OLD-NON-O.pdf