Found the fix - perform install as root, not as user with sudo. It would seem that executing "sudo apt install kix-mysql kix" does not read the variables set with export. Thankfully, I had root access to my online instance (I'm not sure all providers allow that out-of-the-box). Performing the install as root worked perfectly.
Minor gripe. After installation, I find the file /etc/apache2/sites-available/kix.conf exists there, and not in ../conf-available As soon as installation was complete, I wanted to give it a subdomain on my server and enable SSL with Let's Encrypt certbot, and so wanted to set a VirtualHost in sites-available/kix.conf and the pre-existing conf file does not allow (easily) for the addition of <VirtualHost *:80> And so, I moved kix.conf from sites-available to conf-available, activated it (a2enconf kix.conf) and created a new kix.conf in sites-available, copied from 000-default.conf and tweaked.
All works (but I'm a bit puzzled why it still gets served from
www.mydomain.com/kix/index.pl as well as kix.mydomain.com/kix/index.pl but this is not the place to ask)