Set zero tax class to specific item count from defined product categories in Woocommerce

In WooCommerce, if the cart contains 6 or more items from 2 specific categories, then I would like to set a specific tax class (tax-zero) only to these items (not the entire cart, so not change it for the other products).

I use this piece of code which calculates the number of items in cart that are from the 2 categories, but I can't find how to complete it in order to set them with my "tax-zero" tax class.

add_action( 'woocommerce_before_calculate_totals', 'apply_conditionally_taxes', 20, 1 );
function apply_conditionally_taxes( $cart ){

    $item_count   = $cart->get_cart_contents_count();
    $kingcat_count = 0;

    foreach ( $cart->get_cart() as $cart_item ) {
        if ( has_term( 'patisseries', 'product_cat', $cart_item['product_id'] ) or has_term( 'viennoiseries-et-gaufres', 'product_cat', $cart_item['product_id'] ) ) {
            $kingcat_count += $cart_item['quantity'];
            //echo $kingcat_count; 
        }

    }
}

Solution

Code contains: (explanation added as a comment to the code)

Only count the items that belong to a certain category
Set a specific tax class to those items only

function action_woocommerce_before_calculate_totals( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;

    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 ) return;

    /* SETTINGS */

    // Set categories
    $categories = array ( 'patisseries', 'viennoiseries-et-gaufres' );

    // Contains 6 or more items
    $contains_min = 6;

    /* END SETTINGS */

    // Counter
    $counter = 0;

    // Loop trough
    foreach ( $cart->get_cart() as $cart_item ) {
        // Break loop
        if ( $counter >= $contains_min ) {
            break;
        }

        // Has term
        if ( has_term( $categories, 'product_cat', $cart_item['product_id'] ) ) {
            // Add to counter
            $counter += $cart_item['quantity'];
        }
    }

    // Check
    if ( $counter >= $contains_min ) {
        // Loop trough
        foreach( $cart->get_cart() as $cart_item ) {
            // Has term
            if ( has_term( $categories, 'product_cat', $cart_item['product_id'] ) ) {
                // We set "Zero rate" tax class
                $cart_item['data']->set_tax_class( 'Zero rate' );
            }
        }
    }
}   
add_action( 'woocommerce_before_calculate_totals', 'action_woocommerce_before_calculate_totals', 10, 1 );