[Resolved] Insert featured image in content (before last H2 + caption)

Try this method:

function featured_image_after_last_h2($content) {

    // abort if not single post or content has no H2
    if ( ( !is_singular('post') ) || ( strpos($content, '<h2') === FALSE ) ) {  
        return $content; 
    }
    // get the post thumbnail and caption
    global $post;
    $feat_img = get_the_post_thumbnail($post->ID, 'post-single');
    $feat_caption = get_the_post_thumbnail_caption($post->ID);
    
    // set the counter target to last H2
    $heading_count = substr_count($content, '<h2>'); 

    // loop through content and insert the HTML
    $content = explode("<h2", $content);
    $new_content = '';

    for ($i = 0; $i < count($content); $i++) {
        if ($i == $heading_count) {
            $new_content .= '<figure class="incontent-featured-image">' . $feat_img . '<figcaption>' . $feat_caption . '</figure>';
        }
        if($i>0) {
          $new_content.= "<h2".$content[$i];
        } else {
          $new_content.= $content[$i];
        }
    }

    return $new_content;
}
add_filter('the_content', 'featured_image_after_last_h2');

For reference and kudos, i found the code for before H2 here:

https://stackoverflow.com/q/61668699

And then revised that to find the last H2 to insert before 🙂

Glad to hear it is working!