Magento 2: Hide other shipping methods when free shipping is available

Use the extension ShippingTweaks.

I had the same problem.

Remove "Free Shipping" configuration because you don't need it (you already have "Cart Price Rules").

When your customer qualifies for free shipping it happens based on "Flat Rate" not in "Free Shipping".

Write a plugin to disable flat rate shipping method when free shipping is actually enabled based on cart sub total.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\OfflineShipping\Model\Carrier\Flatrate">
        <plugin name="disable-flatrate" type="Vendor\ModuleName\Model\Carrier\Flatrate" sortOrder="1" />
    </type>
</config>

Write a Model class to process sub total validation.

<?php
namespace Vendor\ModuleName\Model\Carrier;

class Flatrate
{

    const XML_PATH_FREE_SHIPPING_SUBTOTAL = "carriers/freeshipping/free_shipping_subtotal";

    /**
     * @var \Magento\Checkout\Model\Session
     */
    protected $_checkoutSession;

    /**
     * @var \Magento\Framework\App\Config\ScopeConfigInterface
     */
    protected $_scopeConfig;

    public function __construct(
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
        \Magento\Store\Model\StoreManagerInterface $storeManager
    ) {
        $this->_storeManager = $storeManager;
        $this->_checkoutSession = $checkoutSession;
        $this->_scopeConfig = $scopeConfig;
    }

    public function afterCollectRates(\Magento\OfflineShipping\Model\Carrier\Flatrate $flatRate, $result)
    {
        $scopeId = $this->_storeManager->getStore()->getId();

        $storeScope = \Magento\Store\Model\ScopeInterface::SCOPE_STORES;

        // Get MOA value from system configuration.
        $freeShippingSubTotal = $this->_scopeConfig->getValue(self::XML_PATH_FREE_SHIPPING_SUBTOTAL, $storeScope, $scopeId);

        // Get cart subtotal from checkout session.
        $baseSubTotal = $this->_checkoutSession->getQuote()->getBaseSubtotal();

        // Validate subtoal should be empty or Zero.
        if(!empty($baseSubTotal) && !empty($freeShippingSubTotal)) {

            if($baseSubTotal >= $freeShippingSubTotal) {
                return false;
            }
        }

        return $result;
    }
}