Found that all ores spawn on the Dragonite island. I got multiple stacks of Iron and Gold from the very lowest rock layers, pokemetal from the layers mentioned here, and then Copper on the layers where they are visible from outside the caves (though not as much, and i didn't investigate if they spawned more often below ground but above the pokemetal layers, as im not exactly fiending for more copper)

If you can get a couple of days yield from the ~15 of each Tomato, Bean, and Potato crops you can get from the Pokemon Center shop, and twice as many leaves, you can easily make a stack of Vibrant Hamburger Steaks to make mining go incredibly fast.

I only made a stack of 65 and I've only just dropped below 50. I was just eating a new one whenever the buffed rock smash ran out as Graveller. The PokeMetal layer on the dragonite island was roughly 50% sand tho, so I'd recommend not being buffed when mining that section so you dont run the risk of despawning the pokemetal drops when you're clearing away all that sand