Can’t override magento contact

I seem to be missing something when it comes to overriding controllers. This is a somewhat recurring question in here, but nothing I’ve found in other questions worked for me, so either they might be a bit outdated or I am missing something obvious.
I am on Magento 2.3.4, trying to override Magento’s module-contact functionality.

Here’s what I’ve done:

Created a new module in /app/code/vendor/module.
Checked that the module shows up in module manager / php magento module:status.
Created etc/di.xml with the following code

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="MagentoContactControllerIndexPost" type="VendorModuleControllerIndexExtendIndex" />
</config>

Created etc/module.xml

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

Created /Controller/Index/ExtendIndex.php with minor functionality for testing

namespace VendorModuleControllerIndex;
class ExtendIndex extends MagentoContactControllerIndexPost
{
    public function execute($coreRoute = null)
    {

    echo "1";
        $this->messageManager->addSuccess('Message from new controller.');
        return parent::execute($coreRoute);
    }
}

Then flushed Magento Cache

Despite all that, which seems to be every step needed for this to work, it hasn’t, and I’ve tried all minor variations in terms of code I could think, as it could simply be a “” somewhere that shouldn’t be be or similar.

What am I missing here?