How do i make the different values of two properties change at the same amount in After Effects?
Pick-whipping one value to another sets it to that value, if you don't want them to be the same but rather change at the same rate then try this instead: 

value + transform.position


Alternatively, parenting the whole camera to a 3D null object may make things easier to animate and is generally nicer to deal with. 

The Angle Script will create a new camera parented to a null for you, which makes things a lot easier: https://madebyloop.co.uk/products/angle-streamline-camera-null-control-after-effects/