Add page title after first block

Question

The only I can think of is to use [parse_block()][1] and [render_block()][1]

parse_block will parse html blocks into an array, and render_block will convert an array into content.

Here is an example of how you can achieve what you want using the above functions (code not tested):

<?php
if (has_post_thumbnail()) {
    the_post_thumbnail();
}
?>

<?php
if (has_post_thumbnail()) {
?>
   <header class="entry-header">
        <?php
            the_title('<h1 class="entry-title">', '</h1>');
        ?>
   </header>
<?php
}
?>

<?php
$renderd_content = "";
$parsed_blocks   = parse_blocks($post->content);
foreach ($parse_blocks as $block_index => $block_attr) {
    if ($block_index === 1 && !has_post_thumbnail()) {
        $renderd_content .= the_title('<h1 class="entry-title">', '</h1>', false);
    }
    $renderd_content .= render_block($block_attr);
}

echo $renderd_content;
?>

Here is a link that provides more examples of this and discuss ACF as well.
https://www.billerickson.net/access-gutenberg-block-data/#acf-block-data