I have installed many replace vehicles without trouble
Yeah, no. See, that's where your thinking took a wrong turn to begin with. Installing 'many replace cars' is NOT a way to circumvent having to do addon cars, but often only exacerbates the situation, as all you're doing is basically have the game use an insane amount of resources on traffic cars. Like a single, fancy addon car can use as many resources as an entire 'vanilla' parking lot! So, turn all those into replace cars, and you're horridly taxing your game to start with. So, no wonder you can only add 3 addon car DLC's.
Also, don't use 1 addon DLC for just 1 car. Consolidate!
And also, get Dilapidated's gameconfig.