member field inside grid field, via add-on (entry model; grid model)

private function create_user_rows($entry, $user_count, $input_id)
 {
     
     $grid_field_id = 20;
     $default_member_id = 123; 
 
     $new_grid_data = [];
 
     for ($i = 0; $i < $user_count; $i++) {
         if ($i == 0) {
             // For the first row, use input_id and fetch the username
             $member_username = ee('Model')->get('Member', $input_id)->first()->username;
    
             $new_grid_data['new_row_' . $i] = [
                 'col_id_58' => $member_username, // Populate with the username of the author
                 'col_id_59' => $input_id,       // Set the relationship field to the author_id
                 'col_id_62' => 'unassigned',    
                 'col_id_60' => 0,          
                 'col_id_61' => 0                 
             ];
         } else {
             // For subsequent rows, use the default member_id
             $new_grid_data['new_row_' . $i] = [
                 'col_id_58' => '',               // No username
                 'col_id_59' => $default_member_id,   // Default member ID for relationship field
                 'col_id_62' => 'unassigned',   
                 'col_id_60' => 0,          
                 'col_id_61' => 0   
             ];
         }
     }
 
     // Load grid model to save grid data
     ee()->load->model('grid_model');
 
     // Save the grid data to the entry using the recommended model-based method
     $entry->{$entry->getCustomFieldPrefix() . $grid_field_id} = $new_grid_data;
     $entry->save();
 
     // Double-save with the grid model to ensure grid data is committed
     ee()->grid_model->save_field_data($new_grid_data, $grid_field_id, 'channel', $entry->entry_id);
 }