I'm "hand drawing" the shape of the slabs, using a tablet. Using Adobe Draw and a stylus, I am drawing the slabs in a grid, with enough "wiggle" to make them uneven but keep them as a strong enough grid for purpose.

But all the styling is photoshop tricks.

Although the slabs take a while, the actual map part - where the walls and the shadows, and the other features - that stuff is quite quick.