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I think a second hand digital camera would be cheaper and easier to use, as well as better quality.

Maybe worth a 3D printed holder like this.

https://www.thingiverse.com/thing:3645894

...and would there be any distortion in the result ? If this works, it should also work for printing sewing patterns which are a pain to copy by hand.

An iPhone can't focus any closer than several centimeters away.

The macro lens on the 13 and up focuses best at about 1.5 cm but it seems to have issues with being out of focus in the corners.

It's called field curvature and even modern macro lenses show some of that.

You need a lens that's designed to reproduce flat subjects to get something decent: https://www.closeuphotography.com/lens-tests

Depends, can you turn off the post-processing?