Severance agreement arrived, need a second opinion after 16 years as a web developer

I was offered 16 weeks of severance after 16 years as a web developer, and I have a few days to sign. This is my first time dealing with any of this. I do not want to leave money on the table. Did anyone negotiate and succeed?

I asked for two more weeks and a neutral reference. They agreed to the reference and added a week.