IMagick is a native PHP extension to create and modify images using the ImageMagick API. ImageMagick Studio LLC did not write nor does it maintain the IMagick extension, however, IMagick users are welcome to discuss the extension here.
I have tested the following which creates a new image and it works fine, so it has something to do with creating a new image versus reading in an SVG, but I can't figure out what.
We wrote the MagickCore / MagickWand API and have not used Imagick. However, if Imagick has analogs to the MagickCore / MagickWand API, it would have a setBackgroundColor() method which you should insert just before the ReadImage() method. Also remove the call to setImageBackgroundColor().