Drupa-based Website Upgrade

+ We have a website that is based on Prosepoint 0.46, Drupal 6 & MoneyScripts module from MoneyScripts.net running on Debian 8.11 (Jessie) with PHP 5.6.40.
+ It’s a membership/subscription only CMS for a newsletter with a handful public pages to introduce the website to guests and allow them to sign-up.
+ We would like to upgrade it to latest Debian (11, Bullseye) and PHP (7.4)
+ Upgrading Drupal to latest version (9.x) is conditional/optional
+ This website is hosted on one of our in-house servers.
+ Upgrading current Debian OS, or fresh installing it, will be done by in-house IT staff.
+ Upgrading current PHP, or fresh installing it, will be done by in-house IT staff.
+ All code modifications, and code additions will be reviewed before acceptance.