Hi Juergen,
I think someone else has mentioned similar issue before and I believe some updates are coming for WP Show Posts. Not sure if it will be included in the free version though.
For now try some CSS to see if we can get it closer:
a.wp-show-posts-read-more,
a.wp-show-posts-read-more:visited
a.wp-show-posts-read-more:hover {
padding: 0;
border: none;
border-radius: 0
}
Adding CSS: https://docs.generatepress.com/article/adding-css/
Let me know if this helps.