Lots of shops across China offer this upgrade service for Apple products.
And some of those shops are also modifying GPU's with up to 48 GB of RAM.
And strangely the 512GB is ALSO using a single NAND chip... Must be a limitation imposed by the A18 Pro. Or just another cost cutting measure. Perhaps both.🤷🏻♂️
I could not find a definitive answer but it does look like the A18 Pro does only support one NAND chip. I could be wrong though.