Shortcut to widget/sidebar editing on the admin menu?

I think that using standard WP featurs there’s no shortcut. But you can workaround using a custom widget.

As example create a very simple widget, something like:

class MyPageWidget extends WP_Widget { 
    function __construct() {
      parent::__construct( 'MyPageWidget', 'My Page Widget' );
    }
    function widget( $args ) {
        $page = get_page_by_path('sidebar-page');
        if ( ! empty($page) ) {
          echo $args['before_widget'];
          echo apply_filters('the_content', $page->post_content);
          echo $args['after_widget'];
        }
    }
}

add_action('widgets_init', function() { register_widget('MyPageWidget'); } );

This few lines of code, add a new widget for your site, with no options, it simply display the content of a page that has the slug 'sidebar-page'.

Now where you have your text widget, replace with this one.

Create a page and assign the slug ‘sidebar-page’.

Now everything you add in that page is displayed in the sidebar: edit a sidebar widget text is simply as edit a page, in effects, it consists in just edit a page.