with the expression, you shouldn't need to link the layer to a null, you can apply the expression to the layer's position property directly, and the layer will rotate around the 'centre' value in the expression.
if you wanted each layer to have a different start position you could add an offset value to the expression, and alter that for each layer.
something like this should work: radius=300;
the offset value would be in degrees, just modify that as needed.
if you wanted to link the 'centre' value to a null, you could change that part of the expression to something like this: centre=thisComp.layer('Null 1').position
where 'Null 1' woudl be the name of your null layer.