Have you considered leaving your rental car in San Diego and taking Amtrak to LA? Less stressful, the Amtrak drops you off at Union Station in downtown LA, which is not a bad place to grab some lunch. (not the best, but not bad

) and Union Station is cool!
Then a $3 Flyaway bus from Union Station to LAX.
This would be my choice, but then again I drive the 405 every day so I am sure as heck not driving it if I am on vacation!