Wrapping my head around multi-level 3D grid based pathfinding

I’m trying to come up with a way to pathfind on multiple levels. See image:

The orange walls would be considered a ladder, connecting you to the upper levels.

At first I thought of looking at it as though it were top down 2d, but I also need to be able to go under things at some point. Does anyone have any suggestions as to where to start? If there is an asset that can handle this out of the box?

Any advice would be welcome

Have you looked at the NavMesh Surface system using NavMesh Links? You can set up wall walking etc rather easily with it- Youtube video