Taking another step back in the process, here's a few images of the material development phase. I had some good advice to take it channel by channel, starting with a pretty clean base color. That said, having fairly uniform colors, roughness, etc. leads to things looking pretty unreal.
Adding color and roughness variation helped a lot, though getting the scale of these details wrong shifted the problem in a different direction instead of fixing it outright. Having some reference is pretty crucial here.
Troubleshooting became important as more and more layers were applied. In particular, the dirt layer wasn't really coming through clearly. Turned out to be the wrong blending mode in the metalness channel leaving those details- who would have known.
Getting feedback along the way helped too. I talked to an engineer who had actual experience with industrial machinery. They had more insight into what happens to certain materials through the course of everyday existence. Wear and tear must have its say.
Anyway, that's it for this particular stage!