strangely familiar to this, but it is different, so here’s a modified version
add_filter( 'post_link', 'remove_parent_cats_from_link', 10, 3 );
function remove_parent_cats_from_link( $permalink, $post, $leavename ){
$cats = get_the_category( $post->ID );
if ( $cats ) {
// Make sure we use the same start cat as the permalink generator
// what happens now actually is the opposite,
// we end up using the latest category that has a parent
usort( $cats, '_usort_terms_by_ID' ); // order by ID
foreach( $cats as $cat ) {
if ( $cat->parent ) {
// If there are parent categories, collect them and pick the top most
$parentcats = explode(" ",get_category_parents( $cat, false, ' ', true ));
$topcat = $parentcats[0];
} else {
$topcat = $cat->slug;
}
}
}
$permalink = home_url()."https://wordpress.stackexchange.com/".$topcat."https://wordpress.stackexchange.com/".$post->post_name;
return $permalink;
}