I'm sorry but this is a little confusing. If I understand the process I should use is as follows:
1.) Original test site: Use Akeeba backup to backup the site resulting in XXX.jpa file
2.) In the live site target / destination root file directory under public_html, place the single line .htaccess file with the ADDHANDLER line in it
3.) Copy the XXX.jpa Akeeba backup file along with kickstart.php to the public_html root directory (so at this point there are 3 files, .htaccess file, XXX.jpa and kickstart.php
4.) Run the recovery process with kickstart.php (domain.com/kickstart.php
This will recover the site in the destination/target directory.
Once the restore process is complete, the cleanup step will rename your restored .htaccess file (from the backup .jpa file) and make it active again. Hopefully all will be correct and the site will now be running PHP 7.3 as it should.
Do I have that correct?