When the party in power gets to decide how the lines are drawn, there is zero incentive to do anything to fix it.

I think the solution is really, really easy. Treat politicians like kids.

One gets to draw the lines, but they have to draw several different versions, that are at least XX% different, in terms of voter groupings. Then the other gets to pick which one gets used.

And when they can't get along, I get to send them to the naughty step.