Have you never been up a mountain? You can go up one very steep portion at 45 degree incline and then come out the other side going on a very long down hill at like 5 degree decline, basically flat.

It doesn't matter. The battery is still regenerating on the way down until you're back at the start point. A less steep angle might even be better because there's no potential to lose energy since the battery can only regen so much at a given time.

On less steep inclines the car still has to do more work to move it’s wheels at a desired speed, even if gravity helps.