It’s not Los Angeles County but try Irvine and Huntington Beach. Had good experiences with both.
HB did my alignment under warranty, no questions asked. Irvine recently did my 2nd oil change with an ADD bumper and they replaced the onboard modem under warranty also no questions asked.
Mere speculation but an issue that you might run into is that since you went to a dealership already, they may have made notes for your vehicle in the system that you’ve tried to get a warranty job done but refused due to aftermarket part so other dealerships may be able to see it. Hopefully they also didn’t red flag your warranty.