A simple “My life has changed to the point I can no longer commit to this project. My last day/gig will be (pick a date).”
I can't. They be screwed. They have nobody who could just step in this weekend and the next. I will do the shows and do them well. I will let them know probably that Sunday or Monday. Gives them 6 weeks to break the new guy in who clearly has been identified. He will do fine but he's has a bunch to learn
It’s short, clean, true, not personal, and no room for rebuttal other than the amount of time you are willing to help.
As for finishing out with these two “Big Gigs”, if you are traumatized by the personalities of the group to quit, it’s not worth the money to stick around. Do yourself a favor, move on now.