How to change the "Read more" link text

For the widget, use the strong_widget_read_more_text filter. For example, add this to your theme’s functions.php:

/**
 * Change the widget "Read more" link text.
 */
function my_strong_widget_read_more_text( $text ) {
	return "More testimonials »";
}
add_filter( 'strong_widget_read_more_text', 'my_strong_widget_read_more_text', 10, 1 );

For the [strong] shortcode, use the more_post and more_text attributes like in this demo.

https://wordpress.org/plugins/strong-testimonials/