Why still output /wp-content/themes/twentynineteen?

You need to deactivate and reactivate your theme.

When a theme is activated the template and stylesheet values are set in the database. If you remove Template: from the child theme’s stylesheet, the template is still set to twentynineteen in the database, so get_template_directory_uri() will return its URL. Re-activating the theme will ensure that this isn’t set to the wrong theme anymore.