" I have to basically get the mental model of the codebase in my head no matter what."

This is a key insight, I'm unable to get around this.

It's the thing I require to have before I let go, and I want to make sure it's easy to grasp again aka clear in the docs.

Basically - the sys architecture, the mental model for key things, even the project structure, you have to have a pretty good feel for.