Any way to Make Linear Keyframe default?

You can create the animation for one image, set the interpolations to Linear, then Copy the selected clip. Then select all your other images and select Paste Attributes (Command-Shift-V) and limit the options to Tranform > Position. Then if you need to customize each animation, all you have to do is modify the keyframed positions. If you're using a different effect than the "basic" Transform, the same thing applies, but just choose the "path" to the Position parameters in the popup dialog.

If you have Motion, you can make an effect that will default to linear. Assuming for a minute you do, what you do is apply the Position X parameter to a Rig slider and the Position Y parameter to a second Rig slider. Set up the sliders to go from a minimum to a maximum (you won't get "infinite" extensibility — at this level — that will require a mulitplier). Basically, just set up your images to go far enough offscreen to be out of bounds - these will be your maximum and minimum rig values. Don't worry about project aspect sizes, it will work in all of the (supported) sizes in FCPX — so if you built it for 1080, it will work just the same in 4K UHD.

If you want to have the option for "smooth" on one of the parameters, publish it first. You only get one! To change the interpolation to smooth, you have to go into the Video Animation editor and select the parameter. You'll get a drop down button on the right edge — clicking it will open the animation keyframe editor. Right-click on the line connecting two keyframed points and select Smooth from the popup.

If you don't have Motion, let me know — I'll set you up with the effect.

If you use the same setup over and over again, you can convert the Transform > Position animations into an Effects Preset, give it a meaningful name and then you will have it to apply to any other clip. You can also mass apply effects presets to every selected clip in the storyline by double clicking it. It's just the matter of going through the effort that one first time (and the Transform, Crop and Distort tools can be "customized" as Effects Presets!)

I posted an effect here:

https://fcpxtemplates.com/ngtopics/LinearTransforms.zip

As I described, it is the Position > X and Position > Y as separate rigged parameters. They are Linear by default. (Only the X parameter can be changed to Ease In, Ease Out or Ease Both. Not something I believe you're worried about ;) .)

The parameters themselves are in % more or less. -100% is actually -1936 (for 1080HD - project size will be scale automatically) to 1936 @100%. Y goes from -1100 to 1100. (0,0) is dead center. These values make an image move just barely offscreen. If your images are an unusual aspect, or oversized, you may have to scale them, crop them, or use the Conform > Fit option. You can also combine this effect with other effects to combine animations. Or, if you have Motion, go in and tweak the rigged parameters to allow for more "travel". If you combine effects, realize that you are not really just moving the image (or clip), you are moving its "viewport" as well. Some really strange cropping effects can (and will) occur if the viewports do not align with the Viewer. [Each layer of effect creates a new viewport referenced to the Viewer (which should be called a Canvas), so whatever is visible at the time of displacement gets moved as is: you don't get your "outside edges" back! until you reset all the parameters back to 0.]

Also, again, if you're using the same animation for all your images, you can create an Effects Preset out of the first one.

Adding effects like this one to clips literally costs nothing. There's no performance hit (or I should say: difference) for something this small. Editing a .plist would be an extreme measure.