with PHP within splits the link

What you are doing isn’t ever going to work the way you want. the_terms() is going to produce a list of complete a tags so you can’t nest or merge it with another one like you are trying to do. And there are no convenient arguments or filters that would allow you to alter that link text. There are some not-so-convenient ones though. The simplest is a filter on get_the_terms

function prefix_term_name($terms, $post_id, $taxonomy ) {
  foreach ($terms as &$term) {
    $term->name="Terug naar ".$term->name;
  }
  return $terms;
}

Which you would use like:

?>
<h3 class="widget-title"><?php
  add_filter( 'get_the_terms','prefix_term_name',10,3);
  the_terms($post->ID, 'fluxus-project-type'); 
  remove_filter( 'get_the_terms','prefix_term_name',10,3); ?>
</h3><?php

The function definition itself can be in your functions.php file to keep things neat.
}