Using full site editing, can I edit just one specific post / page to be unique?

by

In your single template you could include a condition to print a different title

Something like

if( 123 === get_the_ID() ) { // Add your special post ID here ?> 
    <h1>My custom title</h1>
<?php } else { ?>
    <h1><?php the_title(); ?></h1>
<?php }