Custom wp-config for wordpress behind reverse proxy & using a staging server

Wordpress, especially in version 5+, is an amazing piece of software. However, in certain situations the default code falls short. Recently, I needed to set up Wordpress behind a reverse proxy, parallel-host a staging server, and use both a visual page builder and a javascript-based translation plugin. And all of it needed to be easy-to-use for non-technical client marketing staff.