Conditionally change PRODUCT subtotal

Hi guys,
I have an issue of conditionally setting product subtotal.

The task is if 5 products are bought of the same category, 6th added product gets a 100% discount.

I’ve done it using this code:

add_action( 'woocommerce_before_calculate_totals', 'make_nth_cat_item_in_cart_free' );
function make_nth_cat_item_in_cart_free( $cart_object ) {

	if ( is_admin() && ! defined( 'DOING_AJAX' ) )
		return;

	if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 ) 
		return;
 
	// it is our quantity of products in specific product category
	$quantity = 0;

	// Nth category product for free
	$numberOfCategoryProductForFree = 6;
 
	$categories = array('category_1');

	foreach ( $cart_object->get_cart() as $hash => $value ) {

		// If product categories is found
		if ( has_term( $categories, 'product_cat', $value['product_id'] ) ) {
			// if yes, count its quantity
			$quantity += $value['quantity'];

			// Save nth product in category id
			if ($quantity == $numberOfCategoryProductForFree) {
				$freeProductId = $value['data']->get_id();
			}
		}
	}
 
	// change prices
	if( $freeProductId && $quantity >= $numberOfCategoryProductForFree ) {
		foreach ( $cart_object->get_cart() as $hash => $value ) {
			if ($value['data']->get_id() == $freeProductId) {
				if ($value['quantity'] > 1) {

					// HERE IS THE PROBLEM

				} else {
					$newprice = 0;
				}

				$value['data']->set_price( $newprice );
				
				break;
			}
		}
	}
 
}

The problem is when product quantity is > 1 i am unable to set product price to 0, or I do not want to change product price to other amount, I just want to display the product subtotal for 1 quantity less of a price.

Please let me know how can I:
TEST CASES:
1) make sure if 6 quantities of a same product are added to cart, product subtotal is displayed only for 5 product price.
2) make sure if 3 quantities of one product is added and 3 quantities of other product is added I need to make sure second product subtotal would be a price of 2.