Should I negotiate my severance? - what worked for you?

I was offered 18 weeks of severance after 18 years as a customer-service rep, and I have a few days to sign. There are 18 people counting on me, so I am trying to be careful. There is a non-disparagement clause. Should I see a lawyer?

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