What is a good canvas size comparison mockup (a Photoshop action to fill all the smart objects with one image would be awesome too)?

You’ve already hit the nail: using linked Smart Objects, all referring to the same placed image file (which can be any kind of imagery like a JPEG but also another Photoshop file) instead of placing files embedded, solves your situation! Check out the crucial difference in the Place command in the File menu (since version

Copy multiple copies of the Smart Object anywhere in the mockup, and if you change the content of this layer (by changing the original file), all instances will change accordingly. You can also use another linked Smart Object layer with an embedded original as a backdrop in the mockup.

Check out the “Create linked Smart Objects” section in this Adobe Help page:
Work with Smart Objects.

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