Use hook_entity_presave to convert multiple lines in a field to separate values in a multi-value field?

I have a field defined as 'long text' that allows infinite number of values. The difficulty is that users are entering everything in one iteration of the field. So, I need to get the value they have entered, explode it and create separate multi-value field entries. But... my approach isn't working:

function builder2020_entity_presave(EntityInterface $entity){
  switch ($entity->bundle()) {
    case 'ec_topics':
      $count = count($entity->field_ec_subtopics);//check if the user has only used one field
      if ($count == 1){ //user has put everything in one field value
        $values = array(explode('\n',$entity->field_ec_subtopics->value)); create array from the field value
          $entity->field_subtopics->value = $values; //feed the array to the field
      }
      $entity->field_sub_topic_count->value = $count; //set the number of ec_topics
      break;

I 'think' I'm close, but I can't seem to figure out where I'm falling down.

case 'ec_topics':
      $count = count($entity->field_ec_subtopics);
      if ($count == 1){
        $values = $entity->field_ec_subtopics->getValue();
        $content = $values[0]['value'];
        $items = explode(PHP_EOL,$content);
        $count = count($items);
        $entity->field_ec_subtopics = $items;
      }
      $entity->field_sub_topic_count->value = $count;
      break;