admin configuration with automatically add certain product to cart for free

I’m trying to create admin configuration with two SKU’s fields, where later you can choose main product and second product will automatically add to cart for free.
So, I created system.xml file first:

<?xml version="1.0"?> <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <tab id="learning" translate="label" sortOrder="10">
            <label>Learning</label>
        </tab>
        <section id="sku" translate="label" sortOrder="130" showInDefault="1" showInWebsite="1" showInStore="1">
            <class>separator-top</class>
            <label>Stock Keeping Unit</label>
            <tab>learning</tab>
            <resource>Learning_Sku::sku_config</resource>
            <group id="general" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>General Configuration</label>
                <field id="sku_first" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                    <label>SKU №1</label>
                    <source_model>LearningSkuModelConfigProductlist</source_model>
                    <comment>First Stock Keeping Unit.</comment>
                </field>
                <field id="sku_second" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                    <label>SKU №2</label>
                    <source_model>LearningSkuModelConfigProductlist</source_model>
                    <comment>Second Stock Keeping Unit.</comment>
                </field>
            </group>
        </section>
    </system>

Then i created observer:

namespace LearningSkuObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppRequestInterface;

class CustomObserver implements ObserverInterface
{
    protected $product;
    protected $cart;
    protected $formKey;

    public function __construct(
        MagentoCatalogModelProductFactory $product,
        MagentoFrameworkDataFormFormKey $formKey,
        MagentoCheckoutModelCart $cart
    ){
        $this->product = $product;
        $this->formKey = $formKey;
        $this->cart = $cart;
    }
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $items = $this->cart->getQuote()->getAllVisibleItems();
        $isFreeItem = 0;
        $isXItem = 0;

        foreach($items as $item) {
            // X is product id
            if($item->getProductId()=="5"){
                $isXItem = 1;
            }
            // Y is free product id
            if($item->getProductId()=="2"){
                $isFreeItem = 1;
            }
        }

        if(!$isFreeItem && $isXItem) {
            $params = array(
                'form_key' => $this->formKey->getFormKey(),
                'product_id' => 2, //product Id
                'qty'   =>1 //quantity of product
            );
            $product = $this->_product->create()->load(2);
            $this->cart->addProduct($product, $params);
            $this->cart->save();
        }
    }
}

And also events.xml for it:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_submit_all_after">
        <observer name="CustomObserver" instance="LearningSkuObserverCustomObserver"/>
    </event>
</config>

But it’s doesn’t working together. I don’t know if I do that task correctly. If yes, how combine all this code? Thanks a lot for helping!