Question: how to get the shiny charm?
Answer: Once your Galar Pokedex is at 400/400, head to Circhester. Enter the west door of Hotel Ionia, go up the elevator, then enter the room on the far left. You should've spoken to the Director once before now in order to get the Catching Charm, but now you've completed the Pokedex, he'll reward you with the Shiny Charm.