Make sure that you're saving the elements of the effects (expression sliders or other effects that you have in the Effects Control Window) that you want to save as an Animation Preset, and not just the 'Null Layer' in the timeline.
Ok - that is allowing me to save the effects/expressions on each individual layer as a preset, but am I able to save expressions/effects from multiple layers at once?
I'm trying to create this preset in a way that once the effect is added to a layer, the other corresponding layers are added automatically.
Am I going about this the wrong way?
Should I be making a .jsx file instead of an .ffx file?
>Should I be making a .jsx file instead of an .ffx file?
Yes, if you want it to add layers, a script would be the way to go.