How to pass PHP template through shortcode - wordpress

Question

I want to create an overview of my latest blog posts and display this information on a certain part of my homepage. My WordPress theme already has a PHP template in the desired format, which means I would not have to create the actual design/style myself.
As I want this information to be shown in one particular part, I was thinking about creating a shortcode which inserts the information then. I have created shortcodes in WP before, however they always populated simple information.
I am running into the problem now that I do not quite know how I can populate a PHP template via a shortcdode. Before the shortcode function was like similar to this one:
function display_my_shortcode($atts, $content, $tag){
//if called from the 'my_primary_shortcode' shortcode
if($tag == 'my_primary_shortcode'){
return 'This is the primary shortcode';
}
//if called from the 'my_secondary_shortcode' shortcode
else if($tag == 'my_secondary_shortcode'){
return 'This is the secondary shortcode';
}
//default
else{
return 'This is something else';
}
}
add_shortcode('my_primary_shortcode','display_my_shortcode');
add_shortcode('my_secondary_shortcode','display_my_shortcode');
My problem is, that the output of the template is still with php - the template I want to export would look like this:
<div class="blog_holder blog_small_image">
<?php $counter = 0; ?>
<?php if(have_posts()) : while ( have_posts() ) : the_post(); ?>
<?php
if(get_the_category(get_the_ID()) != 'test123')
{
continue;
}
else
{
if($counter < 6){
get_template_part('blog_small_image', 'loop');
$counter++;
}
}
?>
<?php endwhile; ?>
<?php if($qode_options_theme13['pagination'] != "0") : ?>
<?php pagination($wp_query->max_num_pages, $blog_page_range, $paged); ?>
<?php endif; ?>
<?php else: //If no posts are present ?>
<div class="entry">
<p><?php _e('No posts were found.', 'qode'); ?></p>
</div>
<?php endif; ?>
</div>
It should create a list of 5 blog posts of the category 'test123'. How would I include this output into a shortcode though. what would be the "return" value?
Thank you guys so much!