I have photos whose aspect ration are not 1.5, which means they will be cropped when I use 6:4 photography paper.

The important thing to remember is that ImageMagick will keep the aspect ratio unless you tell it not to with the “!” option.

So the command to add a left and right border to an image is this:
magick convert -border 2000x! old.jpg new.jpg

To break that down:
convert the file into a new image file.
2000 add 2000 pixels to left AND right side of image
x! height is not specified here.  The ! tells ImageMagick to not preserve the aspect ratio.

Similarly, in order to add to the top and bottom of the photo, I use this command:
magick convert -border !x2000 old.jpg new.jpg

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