I finally got a chance to fix the user registration. I just made use of the Network Subsite User Registration plugin. There are other posts about making the modifications yourself but this worked and makes things simpler. There was also an issue with the WordPress mailer on Debian. Turns out WordPress uses a modified version of phpmailer that was interfering with Debian supplied (and required) package libphp-phpmailer. Here’s the fix:

*** class.phpmailer.php.orig    2018-12-01 14:09:47.000000000 +0000
--- class.phpmailer.php    2020-01-01 22:00:47.817057652 +0000
***************
*** 1492,1497 ****
--- 1492,1498 ----
      public function getSMTPInstance()
      {
          if (!is_object($this->smtp)) {
+             require_once( 'class.smtp.php' );
              $this->smtp = new SMTP;
          }
          return $this->smtp;

This needs to be applied to the Debian-supplied phpmailer at /usr/share/php/libphp-phpmailer/class.phpmailer.php. A kind soul on this support ticket outlined the fix.

If you run into any issues, please let us know.