Is it possible to do something similar to this? As an alternative, I can remove the items from the list entirely if they have already been chosen, but would prefer to not have to update the dropdown options.
the dropdown cells aren’t bounded by any logic so the only way would be to create a custom solution. Updating the dropdown source is the easiest way if you want to point out remaining options while a user is opening the dropdown list of choices. If you would like to mark a cell that has the same value as another one it is possible (by the afterChange hook), however, it requires the editor to already be closed.