How to compare between two dates in a nested loop?

Question

That date format will be a problem for wp_query because it’s a string in the format d-m-y.

Your meta_query is close but the key needs to be the field name, and the stored date format must be yyyy-mm-dd:

'meta_query' => array(
   array(
     'key' => 'usp-custom-51',
     'value' => array($start, $end),
     'compare' => 'BETWEEN',
     'type' => 'DATE'
   ),
)

If you can’t change your stored date format, it might be easiest to get all posts in the category and do your date filtering with PHP.