Marrying when you have a monster-in-law?

We’re grown adults and my SO’s mother doesn’t like me because I had a child when I was young. My SO loves my child and I dearly. She is incredibly rude telling him that he wastes his free time “babysitting.” We’re all happy and ready to marry but she is not going to be supportive. Do we attempt to both talk to her together ahead of time, or just send her an invite and wait for the blowup? Or elope at the courthouse and then tell her and invite everyone for dinner after? 😅

Edit to add: He has already told her how he loves us and considers my son family etc. many times.