I couldn't find the "default" renderer but it was so easy to recreate it that it's not worth worrying about. Here's the updated code:

  headerName: 'My Group',
  field: 'id',
  cellRendererParams: {
    suppressCount: true,
    innerRenderer: params => {
      if (!params.node.group) {
        // Custom rendering logic for individual rows
        return <GroupColumnCellRenderer {...params} />;
      }
      return `${params.node.key} (${params.node.allChildrenCount})`;
    },
  },
};