I have not found a way to do it directly, but the bottom code seems to work more or less.
function wp4378295342_maybe_add_template() {
if ( ! is_admin() || ! isset( $_GET['post'] ) || '1234' !== $_GET['post'] ) {
// This is not the post/page we want to limit things to.
return false;
}
$post_type_object = get_post_type_object( 'post' );
$post_type_object->template = array(
array( 'core/paragraph', array(
'placeholder' => 'Add Description...',
) ),
);
$post_type_object->template_lock = 'all';
}
add_action( 'init', 'wp4378295342_maybe_add_template' );