How can I update shipping method by using observer and event "sales_quote_collect_totals_before" magento 2?

Question

I want to apply functionality for the customer if customer place order within 72 hrs and subtotal greater than 250 then I want to give free shipping method otherwise if above 72 hrs or subtotal is less than 250 then I do not want to show free shipping. so how can I do this? I have done following coding for this functionality. My free-shipping method is not disabled when I add the product to the cart. So How can I disable free-shipping or enable free-shipping withing this condition.

namespace CustomCheckout\Checkout\Observer;

use Magento\Framework\Event\ObserverInterface;

class Totalsbefore implements ObserverInterface
{
    /**
     * custom event handler
     *
     * @param \Magento\Framework\Event\Observer $observer
     * @return void
     */

 public function execute(\Magento\Framework\Event\Observer $observer)
    {   

 $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$customerSession = $objectManager->create('Magento\Customer\Model\Session');

if ($customerSession->isLoggedIn()) {

    $customer_id = $customerSession->getCustomer()->getId(); 
 $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
 $orderCollection = $objectManager->create('\Magento\Sales\Model\ResourceModel\Order\Collection');
$order_Collection = $orderCollection->addAttributeToFilter('customer_id',$customer_id)
                    ->addFieldToSelect('created_at')
                    ->addAttributeToFilter('subtotal', array('gteq' => 250))
                    ->setPageSize(1)                    
                    ->setOrder('created_at', 'desc'); 

                    if(count($order_Collection)>0)
                     {

                         foreach($order_Collection as $order)
                        {
                         $LastOrderDate = $order->getCreatedAt();                    
                        }

                      $LastOrder = strtotime($LastOrderDate);
                       $NextTime = date("Y-m-d H:i:s", strtotime('-1 hours'));
                      $CheckTime = strtotime($NextTime);
                      $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
                      $cart = $objectManager->get('\Magento\Checkout\Model\Cart')->getQuote(); 
                      $subTotal = $cart->getSubtotal();
                      $store  =  $cart->getStoreId();
                      $carriers = $objectManager->get('Magento\Framework\App\Config\ScopeConfigInterface')->getValue('carriers');

                      if($LastOrder>=$CheckTime && $subTotal<=250)
                      {
                        $hiddenMethodCode   = 'freeshipping'; 

                         foreach ($carriers as $carrierCode => $carrierConfig) 
                        { 
                             $hiddenMethodCode   = 'freeshipping'; 
                           if( $carrierCode !=  $hiddenMethodCode)
                            {

                              //  $store->saveConfig("carriers/{$carrierCode}/active", '0');

                                 $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); // Instance of object manager
                              $resource = $objectManager->get('Magento\Framework\App\ResourceConnection');
                              $connection = $resource->getConnection();
                              $tableName = $resource->getTableName('core_config_data'); //gives table name with prefix

                              //Update Data into table
                              $sql = "Update " . $tableName . " Set  value= 0 where path='carriers/{$carrierCode}/active'";

                             // print_r($sql);  
                              $connection->query($sql); 
                            }
                        }          
                      }else{   // < 72 && > 250

                         $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
                      $cart = $objectManager->get('\Magento\Checkout\Model\Cart')->getQuote(); 
                      $subTotal = $cart->getSubtotal();
                      $store  =  $cart->getStoreId();
                      $carriers = $objectManager->get('Magento\Framework\App\Config\ScopeConfigInterface')->getValue('carriers');

                          $hiddenMethodCode   = 'freeshipping'; 

                          foreach ($carriers as $carrierCode => $carrierConfig) 
                          {
                            if( $carrierCode ==  $hiddenMethodCode)
                            {

                              $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); // Instance of object manager
                              $resource = $objectManager->get('Magento\Framework\App\ResourceConnection');
                              $connection = $resource->getConnection();
                              $tableName = $resource->getTableName('core_config_data'); //gives table name with prefix

                              //Update Data into table
                              $sql = "Update " . $tableName . " Set  value= 0 where path='carriers/{$carrierCode}/active'";

                               $connection->query($sql);
                             }
                          }
                        } 

                       }else{         

                      $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
                      $cart = $objectManager->get('\Magento\Checkout\Model\Cart')->getQuote(); 
                      $subTotal = $cart->getSubtotal();
                      $store  =  $cart->getStoreId();
                      $carriers = $objectManager->get('Magento\Framework\App\Config\ScopeConfigInterface')->getValue('carriers');

                              $hiddenMethodCode   = 'freeshipping'; 

                              foreach ($carriers as $carrierCode => $carrierConfig) 
                              {
                                if( $carrierCode ==  $hiddenMethodCode)
                                {

                              //  $store->saveConfig("carriers/{$carrierCode}/active", '0');

                              $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); // Instance of object manager
                              $resource = $objectManager->get('Magento\Framework\App\ResourceConnection');
                              $connection = $resource->getConnection();
                              $tableName = $resource->getTableName('core_config_data'); //gives table name with prefix

                              //Update Data into table
                              $sql = "Update " . $tableName . " Set  value= 0 where path='carriers/{$carrierCode}/active'";

                             // print_r($sql);  
                              $connection->query($sql);
                                }
                              }           
                            }

                    }   
  }  

}

Answer 1

Assuming you have created "My free-shipping" , you would have a file

MyVendor\ShippingMethod\Model\Carrier\MyFreeShipping.php and in collectRates function

 public function collectRates(RateRequest $request)
        {
            if (!$this->getConfigFlag('active')) {
                return false;
            }
            $result = $this->_rateResultFactory->create();
            $method = $this->_rateMethodFactory->create();


            if (!$this->shippingData->isFreeDeliveryAvailable()) {
                return false;
            }


            $method->setCarrier($this->_code);
            $method->setCarrierTitle($this->getConfigData('title'));

            $method->setMethod($this->_code);
            $method->setMethodTitle($this->getConfigData('name'));
            $method->setPrice('0');
            $method->setMethodDescription($deliveryDateMessage);
            $method->setCost('0');

            $result->append($method);

            return $result;
        }
public function isFreeDeliveryAvailable()
{
///check for your conditions here and return true or false
}

I have not tested this code , please let me know if face any error or issue .

This would be a better approach than event observer.