More light is perhaps needed…
You could try bidirectional instead of path. Bi-directional takes a little more time, but it discovers more candidates for lighting, so it is more efficient in the long run. Expecially if you are going to commit 5 hours anyway.
The thing about metals is that they are reflecting the world, like glass. You might want to add something off camera to show up in the reflection.
Is that HDRi lighting?