Implement inputargs in GenerateImageDerivative and GenerateImageDerivativeFile, instead of in AbstractGenerateDerivative. Add utility function to add a form element (or other array entry) at a specific position.
// adjust args title and description for better clarity
$form['inputargs']['#description'] = $this->t('Additional input options for ImageMagick convert (e.g. -density 144).<br>Check the <atarget="_blank"href="https://manpages.ubuntu.com/manpages/trusty/man1/convert.im6.1.html">man page</a> to see which options are input options.');
'#description' => $this->t('Additional input options for ImageMagick convert (e.g. -density 144).<br>Check the <atarget="_blank"href="https://manpages.ubuntu.com/manpages/trusty/man1/convert.im6.1.html">man page</a> to see which options are input options.'),
// adjust args title and description for better clarity
$form['inputargs']['#description'] = $this->t('Additional input options for ImageMagick convert (e.g. -density 144).<br>Check the <atarget="_blank"href="https://manpages.ubuntu.com/manpages/trusty/man1/convert.im6.1.html">man page</a> to see which options are input options.');
'#description' => $this->t('Additional input options for ImageMagick convert (e.g. -density 144).<br>Check the <atarget="_blank"href="https://manpages.ubuntu.com/manpages/trusty/man1/convert.im6.1.html">man page</a> to see which options are input options.'),