I am really sorry, because i am only novice BP and when i try to show you something, i bet there is a better way, to do that.
My english is to limited, to teach someone more then breathing with it.
Here i load the target points in my case into Array.
And here i am extracting them again, but i use random locations. You could check the distance to the next point and choose the next aviable to start track.
I think in your case it would be best to setup a targetpoint grid (at every crossing section, or where the pathdirection changes, but you have to try it out on your own).
Not to complicated i think.
You can use the navpoint / route system that is in the survival game sample:
But there you have to place the routepoints manually.
If you want to go for a more realistic automated route, i would suggest to keep a minimum distance to the borders of the navmesh, like 1 meter for example.
As a human you would go the original red line from A to B with some distance to the walls.
If you want more of a robot/drone behaviour that really goes in the middle of the paths, then i would set up manually a mesh of navpoints, its quick to do.