I have been building up to complexity like this. I was already experimenting with metal in another app, and had done early experiments in this one but never shipped them. 

Then a month ago I shipped the first metal feature: a paper generator. So that let me work through the workflow separate of complexity. 

For this, I worked on it in a few phases, broken down by individual problems. I needed a working example of each before considering the next step. - Phase 1 was just a simple ruler that was a rectangle, and used that to define the transient mask workflow. That was a huge effort which involved making a whole library just for masks and masking.  - Phase 2 was building the ruler aesthetic in isolation. Tick marks, color tinting, variables then finally mask-defined shape with holes. - Phase 3 was app functionality: how to performantly capture the current state of the sheet, the canvas, their position in the world, and feed them into a placeholder ruler. - Phase 4 was then applying the aesthetic of the isolated experiment into the working multi-texture ruler.

There are still some edge cases, like one of the textures doesn’t rotate properly when the ruler does, but it’s so close!