If you want to apply it to ALL WPSP lists, you can remove the section#wpsp-xxxx
part.
But if you want to add more, you add extra selectors.
Example:
section#wpsp-302 .wp-show-posts-inner .wpsp-read-more,
section#wpsp-123 .wp-show-posts-inner .wpsp-read-more,
section#wpsp-456 .wp-show-posts-inner .wpsp-read-more {
margin-top: auto;
}
section#wpsp-302 .wp-show-posts-inner,
section#wpsp-123 .wp-show-posts-inner,
section#wpsp-456 .wp-show-posts-inner {
display: flex;
flex-direction: column;
}