Unfortunately I cannot test on Windows. A quick search suggests that maybe the command on Windows should be:
Code:
magick convert path/to/input/image -quality 80 -colorspace gray /path/to/output/image
If the above command does not work on Windows, then the problem is not in the plugin and you have to figure the right command yourself.
If the command runs successfully on your machine, run it in Editor Chains and post the debug log. You can run the editor in debug mode using the following command:
Code:
calibre-debug --edit-book