In theory, neutral experts could be used to determine the best policies towards the democratically chosen goals. And it actually works that way for the most part. The vast majority of stuff that goes through a parliament is stuff you never hear about, after all. They're like your car example. It's the few percent of topics where there's controversy where it's difficult, in practice, to find experts who will genuinely stay neutral - precisely because they're so controversial.
I disagree with that. If you asked people what the goals should be for educating children, they’d probably be in broad agreement. But they differ widely in how they think those goals should be achieved.
Experts are only useful when there is a fairly rigorous scientific framework in place that has predictive power—the ability to predict what will work and what won’t.
Even in areas adjacent to those fields, the effectiveness of expertise quickly breaks down. Public health has elements of both medicine and social psychology, and the results were all over the place during the recent pandemic. When you get into other territory: education, welfare, etc., the real world value of expertise is even less.
No, we have democracy in order to choose between competing policies and ideologies.
An expert comission has a pre-defined, fixed policy - produce fair district maps, and they work to carry it out. The idea that no-one can carry out a task without bending the result to suit their personal views is absurd. If you ask some car designers to design you a saloon car, they'll design a saloon car, even if they personally prefer sports cars.