What I can recommend is getting Naviserver in a docker container with the latest Operating System underneath it, get PostgreSQL 8.4 from the docker hub container and then run all of this in a new environment.
This will get around the issue / incompatibility of newest OS and PG 8.4 while keeping your environment secur(ish) as anything you expose to the outside runs on the latest Versions of the OS.
We did this with multiple old systems where we did not touch the Database or the OpenACS code.
You will run into more warnings in the Error.log due to AOLserver calls being deprecated in Naviserver, but these are few and you can easily use SED on your code to search and replace those instances (as you already mentioned your code is so heavily customized it does not make sense for you to Upgrade OpenACS, so this additional bit of code change wont make a difference I'd wager.
