Thanks again Kaspars!
In my case I’ve implemented your suggestion like so:

if ( is_array( $table['data'] ) ) {
        $form_data = WPCF7_Submission::get_instance()->get_posted_data();

	//Grab the arch field
	$arch = WPCF7_Submission::get_instance()->get_posted_data( 'arch' );
	// Cast to array and join the array elements with a comma.
	$arch = implode( ', ', (array) $arch );
	// Write new value to form_data
	$form_data['arch'] = $arch;
	
	$table['data'][] = array_map( 'strval', $form_data );
	$table_model->save( $table );
}

For now i’m grabbing the specific fields that I know need to be imploded, some dropdowns and a group of checkboxes. I’m wondering, if I’m not using the ', ' separator can I completely remove the first argument? My temp solution is to make it an empty ''.