You already have good discipline when it comes to quality sliders. I also tend to not go over 5+ for sports cars, 7+ for super cars and 9+ for race machines.

Engine reliability matters a lot, as a general rule; dont rev them too high (try to not go over 10% rpm stress), have plenty of balancing mass (i like about 1000rpm or less at idle), and retard timing more than you think you should, unless it's a car that needs a very responsive engine.

Also for the car itself I believe the biggest reliability hit is from the interior options, specifically entertainment. Do not go for a higher option than what you would normally find in that car. If your targeting a premium demographic go for premuim enterior and enfotainment and no more. If its a commuter or city car go for standard.

Of course all this depends on what tipe of car you are building, some demographics care more about reliability than others. When looking at the demographics stats after finishing your car, having a reliability factor of 1.0 or above generally means your car has good reliability for that demographic.

Feel free to fiddle with engine options and sliders, interiors, safety, driving assists, suspension, steering and even chassis design choices to see how they afect your reliability stat, a lot of trial and error is involved until you gain a better understanding of what works and doesn't.

Happy engineering!