switch( $_GET['orderby'] ) {

  case 'eval_workshop_date':
    $mq = array(
        'relation' => 'OR',

        'individual' => array(
            'key' => 'workshop_or_individual_clients',
            'value' => 'A workshop',
            'compare' => '!='
        ),

        'workshop_date' => array(
            'key' => 'workshop_completion_date',
            'value' => '',
            'compare' => '!='
        ),
    );

    // Add existing meta query back in if it exists
    if ( $query->get( 'meta_query' ) ) {
        $mq = array(
            'relation' => 'AND',
            $mq,
            $query->get( 'meta_query' )
        );
    }

    // Set the meta query
    $query->set( 'meta_query', $mq );

    // Change it to order by our new meta query options
    $query->set( 'orderby', array(
        'individual' => 'DESC',
        'workshop_date' => 'DESC'
    ));
  break;
}

SELECT DISTINCT SQL_CALC_FOUND_ROWS wp_posts.*
FROM   wp_posts
       INNER JOIN wp_postmeta
               ON ( wp_posts.ID = wp_postmeta.post_id )
       INNER JOIN wp_postmeta AS mt1
               ON ( wp_posts.ID = mt1.post_id )
       INNER JOIN wp_postmeta AS a
               ON ( a.post_id = wp_posts.ID )
       INNER JOIN wp_postmeta AS b
               ON ( ( b.meta_id = a.meta_id + @@auto_increment_increment )
                    AND ( b.post_id = wp_posts.ID ) )
       INNER JOIN wp_posts AS c
               ON ( ( c.post_type = 'acf-field' )
                    AND ( ( c.post_content LIKE '%:"text"%' )
                           OR ( c.post_content LIKE '%:"textarea"%' )
                           OR ( c.post_content LIKE '%:"wysiwyg"%' ) ) )
WHERE  1 = 1
       AND ( ( ( c.post_name = b.meta_value )
               AND ( a.meta_value LIKE '%Workshop%' )
               AND ( a.meta_value LIKE '%5%' ) )
              OR ( ( ( wp_posts.post_title LIKE '%Workshop%' )
                      OR ( wp_posts.post_content LIKE '%Workshop%' )
                      OR ( wp_posts.post_excerpt LIKE '%Workshop%' ) )
                   AND ( ( wp_posts.post_title LIKE '%5%' )
                          OR ( wp_posts.post_content LIKE '%5%' )
                          OR ( wp_posts.post_excerpt LIKE '%5%' ) ) ) )
       AND ( ( wp_postmeta.meta_key = 'workshop_or_individual_clients'
               AND wp_postmeta.meta_value != 'A workshop' )
              OR ( mt1.meta_key = 'workshop_completion_date'
                   AND mt1.meta_value != '' ) )
       AND wp_posts.post_type = 'dtl-evaluation'
       AND ( wp_posts.post_status = 'publish'
              OR wp_posts.post_status = 'acf-disabled'
              OR wp_posts.post_status = 'future'
              OR wp_posts.post_status = 'draft'
              OR wp_posts.post_status = 'pending'
              OR wp_posts.post_status = 'private' )
GROUP  BY wp_posts.ID
ORDER  BY CAST(wp_postmeta.meta_value AS CHAR) DESC,
          CAST(mt1.meta_value AS CHAR) DESC,
          wp_posts.post_date DESC
LIMIT  0, 20

+-----------------------+------------------------------------+---------+
| Column                | Type                               | Comment |
+-----------------------+------------------------------------+---------+
| ID                    | bigint(20) unsigned Auto Increment |         |
| post_author           | bigint(20) unsigned [0]            |         |
| post_date             | datetime [0000-00-00 00:00:00]     |         |
| post_date_gmt         | datetime [0000-00-00 00:00:00]     |         |
| post_content          | longtext                           |         |
| post_title            | text                               |         |
| post_excerpt          | text                               |         |
| post_status           | varchar(20) [publish]              |         |
| comment_status        | varchar(20) [open]                 |         |
| ping_status           | varchar(20) [open]                 |         |
| post_password         | varchar(255) []                    |         |
| post_name             | varchar(200) []                    |         |
| to_ping               | text                               |         |
| pinged                | text                               |         |
| post_modified         | datetime [0000-00-00 00:00:00]     |         |
| post_modified_gmt     | datetime [0000-00-00 00:00:00]     |         |
| post_content_filtered | longtext                           |         |
| post_parent           | bigint(20) unsigned [0]            |         |
| guid                  | varchar(255) []                    |         |
| menu_order            | int(11) [0]                        |         |
| post_type             | varchar(20) [post]                 |         |
| post_mime_type        | varchar(100) []                    |         |
| comment_count         | bigint(20) [0]                     |         |
+-----------------------+------------------------------------+---------+

Keys:
+---------+---------------------------------------+
| PRIMARY | ID                                    |
+---------+---------------------------------------+
| INDEX   | post_name(191)                        |
| INDEX   | post_type, post_status, post_date, ID |
| INDEX   | post_parent                           |
| INDEX   | post_author                           |
+---------+---------------------------------------+