Different shipping classes - wordpress

Question

I have a website where I need to charge different delivery fees for different products.
The standard delivery fee is $50 per order irrelevant of how many items.
However, I have some bulky orders where I will need to charge $150 for them and up to 2 items. So let's say if you buy 2 bulky items, the delivery charge would be $150 but if you buy 3 it would go up to $300 and so on.
When the client puts items in the cart, they will be charged for bulky items and regular items separately, so if the customer puts 1 bulky item and 1 normal item, they need to be charged $150 + $50 which makes the delivery fee $200 in total.
I tried to create a shipping class for the bulky items and in the settings, I have configured it as below:
I also have this logic in my functions file which takes care of charging in multiples in case of heavy products:
add_filter( 'woocommerce_package_rates', 'different_rates_based_on_quantity_steps', 10, 2 );
function different_rates_based_on_quantity_steps ( $rates, $package ){
$slug_class_shipping = 'test'; // Slug of the shipping class
$step_change = 2; // Steps for item
$item_count = 0;
// Checking in cart items
foreach( WC()->cart->get_cart() as $cart_item ){
// If we find the shipping class
if( $cart_item['data']->get_shipping_class() == $slug_class_shipping ){
$item_count += $cart_item['quantity']; // Cart item count
$rate_operand = ceil( $item_count / $step_change ); // Operand increase each # items here
foreach ( $rates as $rate_key => $rate ){
// Targetting "Flat rate"
if( 'flat_rate' === $rate->method_id ) {
$has_taxes = false;
// Set the new cost
$rates[$rate_key]->cost = $rate->cost * $rate_operand;
// Taxes rate cost (if enabled)
foreach ($rates[$rate_key]->taxes as $key => $tax){
if( $tax > 0 ){
// New tax calculated cost
$taxes[$key] = $tax * $rate_operand;
$has_taxes = true;
}
}
// Set new taxes cost
if( $has_taxes )
$rates[$rate_key]->taxes = $taxes;
}
}
return $rates;
}
break;
}
}
That said, unfortunately, this is not working as expected.
If I add 2 bulky items, this is calculating the delivery fee at $200 (instead of $150), and if I add regular items the delivery fee doesn't change and remains at $200.
If I add 3 bulky items, delivery is totalling $400 (instead of $300) and if I add 5 it is going up to $600 (instead of $450). It is like always adding the base fee in case of bulky items, but not adding it in case of regular ones.
Can someone help me with what I'm doing wrong?