Show thumbnails for posts, but not for the post your on

Outside the loop

<?php $postid = $post->ID; ?>

Inside the loop

<?php if ( ($post->ID != $postid ) && has_post_thumbnail() ) :?>

HTH

David

https://codex.www.ads-software.com/Function_Reference/WP_Query#Post_.26_Page_Parameters

'exclude' might not be the right/valid parameter; try 'post__not_in':

change from this:
'exclude' => $post->ID
to this:
'post__not_in' => array($post->ID)

—
to get only posts with thumbnails, you can try and add this to the query parameters:
'meta_key' => '_thumbnail_id'

(or try:
'meta_query' => array(array('key' => '_thumbnail_id'))
(untested)
https://codex.www.ads-software.com/Function_Reference/WP_Query#Custom_Field_Parameters
)