I'm not sure is Monarch support set up inside package project?
I found a problem that monarch can't find assets.
In my widget I load assets with my package name
SvgPicture.asset( 'assets/images/header_bg.svg', package: 'my_package', ),
I found a work around to make It work by copy files from '.monarch/flutter_assets/assets/images' to '.monarch/flutter_assets/packages/my_package/assets/images'