I have the same question, and I already have the Pro version. Is there any way simpler than the [wpdm_frontend] shortcode, perhaps customizing the page template? I need the user with whom a package is shared to be able to add to the package by uploading a file.
Yes, it is possible to customize the frontend uploader form using custom templates.
Frontend uploader uses the /tpls/wpdm-add-new-file-front.php template to show the form. You can make your own copy of this template to simplify the uploader. Please check the template customization doc here https://www.wpdownloadmanager.com/doc/template-files/
There is another shortcode [wpdm_package_form] which only shows the uploader form instead of all things shown by the [wpdm_frontend] shortcode. You can check that out too.