Adding additional advert after set number of paragraph

I've currently got my website set up where it automatically adds a Google Adsense ad after the 2nd paragraph of any article, but I'd like to improve on this, if anyone is able to help.

I'd like to add to this code to add another 2 adverts; one after a 6th paragraph and another after a 10th. If the article doesn't reach these paragraph numbers then the adverts shouldn't show.

It's probably something really obvious but anything I've tried has resulted in the functions.php file crashing when I reload the site.

My code is...

add_filter( 'the_content', 'prefix_insert_post_ads' );

function prefix_insert_post_ads( $content ) {

$ad_code = '<div class="mobilead .visible-xs-block hidden-sm hidden-md hidden-lg"><script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
(adsbygoogle = window.adsbygoogle || []).push({}); ';

if ( is_single() && ! is_admin() ) {
return prefix_insert_after_paragraph( $ad_code, 2, $content );
}

return $content;
}

// Parent Function that makes the magic happen

function prefix_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
$closing_p = '</p>';
$paragraphs = explode( $closing_p, $content );
foreach ($paragraphs as $index => $paragraph) {

if ( trim( $paragraph ) ) {
    $paragraphs[$index] .= $closing_p;
}

if ( $paragraph_id == $index + 1 ) {
    $paragraphs[$index] .= $insertion;
}
}

return implode( '', $paragraphs );
}

As a supplementary question - is there a way to limit these ads to showing only on posts, rather than pages as well? Currently they're showing anywhere.

Any help would be hugely appreciated.