You are trying to cheat physics. You have a linear conveyor, so you have to move linear.
When you want to move ptp not hitting the conveyor, you have to calculate intermediate positions above the conveyor. But the position should not be too much above the conveyor. After all, when you have done your own calculations for the intermediate positions you get nearly a linear movement, and the same limitations like standard linear interpolation. That's because during the complete movement some of the axis have to accelerate and deccelerate more than once. On ptp movements each axis only accelerates and deccelerates once.