Show similar category portfolio items?

Question

Here’s what I would do:

I would create a new WP_Query instance, using a Taxonomy parameter as follows:

$wp_query = new WP_Query([
    'post_type'       => '<custom_post_type>',
    '<taxonomy_slug>' => '<term_slug>',
    'fields'          => 'ids', // We just need the post IDs
    // Add other query parameters as needed
]);

More on using Taxonomy parameters in a WP_Query instance

Then, I would loop through the posts and get the post thumbnail as follows:

$post_thumbnails = [];

foreach ($wp_query->posts as $post_id) {
    $post_thumbnails[$post_id] = get_the_post_thumbnail($post_id[, $size="post-thumbnail", $attr = NULL]); // Notice the optional arguments
}

More on the get_the_post_thumbnail() function

Then all you need to do is loop through the thumbnails and display them.

(You’ll probably also want more information associated with the post than just the post ID and it’s thumbnail, so you might want to restructure the query to return more than just the ID.)

Hopefully that answers your question. Cheers!