Magento – Contact form after submission not redirecting to cms page Magento 2

Contact Post Controller needs to be overridden here, because by default it redirects to contact/index
$this->_redirect('contact/index'); needs to be changed to $this->_redirect('cms-page-url-key'); in file vendor/magento/module-contact/Controller/Index/Post.php

Code for controller override is as follows :

app/code/Custom/Contact/registration.php

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Custom_Contact',
    __DIR__
);

app/code/Custom/Contact/etc/module.xml

<?xml version="1.0" encoding="UTF-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Custom_Contact" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Contact"/>
        </sequence>
    </module>
</config>

app/code/Custom/Contact/etc/di.xml

<?xml version="1.0" encoding="UTF-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Contact\Controller\Index\Post" type="Custom\Contact\Controller\Index\Post" />
</config>

app/code/Custom/Contact/Controller/Index/Post.php

<?php
namespace Custom\Contact\Controller\Index;

use Magento\Framework\App\Request\DataPersistorInterface;
use Magento\Framework\App\ObjectManager;

class Post extends \Magento\Contact\Controller\Index\Post
{
    /**
     * @var DataPersistorInterface
     */
    private $dataPersistor;

    /**
     * Post user question
     *
     * @return void
     * @throws \Exception
     */
    public function execute()
    {
        $post = $this->getRequest()->getPostValue();
        if (!$post) {
            $this->_redirect('*/*/');
            return;
        }

        $this->inlineTranslation->suspend();
        try {
            $postObject = new \Magento\Framework\DataObject();
            $postObject->setData($post);

            $error = false;

            if (!\Zend_Validate::is(trim($post['name']), 'NotEmpty')) {
                $error = true;
            }
            if (!\Zend_Validate::is(trim($post['comment']), 'NotEmpty')) {
                $error = true;
            }
            if (!\Zend_Validate::is(trim($post['email']), 'EmailAddress')) {
                $error = true;
            }
            if (\Zend_Validate::is(trim($post['hideit']), 'NotEmpty')) {
                $error = true;
            }
            if ($error) {
                throw new \Exception();
            }

            $storeScope = \Magento\Store\Model\ScopeInterface::SCOPE_STORE;
            $transport = $this->_transportBuilder
                ->setTemplateIdentifier($this->scopeConfig->getValue(self::XML_PATH_EMAIL_TEMPLATE, $storeScope))
                ->setTemplateOptions(
                    [
                        'area' => \Magento\Backend\App\Area\FrontNameResolver::AREA_CODE,
                        'store' => \Magento\Store\Model\Store::DEFAULT_STORE_ID,
                    ]
                )
                ->setTemplateVars(['data' => $postObject])
                ->setFrom($this->scopeConfig->getValue(self::XML_PATH_EMAIL_SENDER, $storeScope))
                ->addTo($this->scopeConfig->getValue(self::XML_PATH_EMAIL_RECIPIENT, $storeScope))
                ->setReplyTo($post['email'])
                ->getTransport();

            $transport->sendMessage();
            $this->inlineTranslation->resume();
            $this->messageManager->addSuccess(
                __('Thanks for contacting us with your comments and questions. We\'ll respond to you very soon.')
            );
            $this->getDataPersistor()->clear('contact_us');
            $this->_redirect('contact-us');
            return;
        } catch (\Exception $e) {
            $this->inlineTranslation->resume();
            $this->messageManager->addError(
                __('We can\'t process your request right now. Sorry, that\'s all we know.')
            );
            $this->getDataPersistor()->set('contact_us', $post);
            $this->_redirect('contact-us');
            return;
        }
    }

    /**
     * Get Data Persistor
     *
     * @return DataPersistorInterface
     */
    private function getDataPersistor()
    {
        if ($this->dataPersistor === null) {
            $this->dataPersistor = ObjectManager::getInstance()
                ->get(DataPersistorInterface::class);
        }

        return $this->dataPersistor;
    }

}

Here contact-us is the cms page url key.