You're making the same mistake again: the rocky coastline of a volcanic island in the middle of an ocean does not become sedimentary. It is constantly being ero

You're making the same mistake again: the rocky coastline of a volcanic island in the middle of an ocean does not become sedimentary. It is constantly being eroded. The plasticrust that is not ingested/incorporated by molluscs and seaweed will be eroded by more waves and be redistributed as molecules in the ocean water. If the island is tropical, a small amount of plastic may get stuck long enough in or to the rocks for coral to grow around it and trap it. But this is probably an insignificant amount compared to all the plastic that is being released and broken down in the ecosystem.