You can use the get_the_excerpt filter to modify what’s printed by the_excerpt():
add_filter( 'get_the_excerpt', 'wpse_367505_excerpt' );
function wpse_367505_excerpt( $excerpt ) {
return substr( $excerpt, 0, 50 ) . ' [Read More]';
}
You can change the [Read More] text to whatever you’d like.
Add this code to your theme’s functions.php file, and it should automatically filter your excerpts anywhere you use them.