Sure, I should have included this information originally. I'm testing this with Apache/2.4.54 (Unix) OpenSSL/1.1.1q PHP/8.1.9 (libphp) and MySQL 8.0.28 - all required PHP extensions enabled.
I have just got the upgrade to go through by editing getDBVersion() to return the schema_signature value from the config table - removing the "SELECT ostversion" query. Upgrade script then ran through fine without any warnings - undid the change to class.config.php - all seems to be working ok.
I since had one warning when attempting a test email send - "Call to undefined function Laminas\Mail\Header\iconv_mime_decode()" - I've since added the iconv PHP extension and will retry the test send shortly.
After restoring the database back to latest 1.16.3 schema - I do end up with the same PHP/mysqli unknown column error when attempting to use the upgrade script. To proceed I had to make the same change in config.class.php to get to 1.17-RC3 a second time.
Thanks for taking a look - happy to provide any further information as required.