If your car airconditioner is acting up, you don't get new tires.
It's the same with the Phantom.
If the compass system has a problem, you look into the compass system - not everything else.
There isn't much that can go wrong with the compass.
Usually it works or it's disconnected.
At worst, if you have to replace yours, a new one is cheap.
To eliminate some obvious things .... if you put the Phantom down on a steel or reinforced concrete surface, the compass will complain and you'll have a compass error.
Taking the Phantom away from the steel makes the compass error away.
Is there any chance your compass error is because the Phantom is near some steel or an electric cable?
DO the numbers in the compass data change when you move the Phantom?
The main number of interest is the Mod value.
A normal value is somewhere around 1500 so yours isn't far out.
What range do you see when you move it?