Show custom currency symbol in Magento2

You need add "symbol" in $currencyOptions array. Please add this below code in your observer file :

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Vendor\Module\Observer;

use Magento\Framework\Locale\Currency;
use Magento\Framework\Event\ObserverInterface;

class CurrencyDisplayOptions implements ObserverInterface
{
    /**
     * @var \Magento\CurrencySymbol\Model\System\CurrencysymbolFactory
     */
    protected $symbolFactory;

    /**
     * @param \Magento\CurrencySymbol\Model\System\CurrencysymbolFactory $symbolFactory
     */
    public function __construct(\Magento\CurrencySymbol\Model\System\CurrencysymbolFactory $symbolFactory)
    {
        $this->symbolFactory = $symbolFactory;
    }

    /**
     * Generate options for currency displaying with custom currency symbol
     *
     * @param \Magento\Framework\Event\Observer $observer
     * @return $this
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $baseCode = $observer->getEvent()->getBaseCode();
        $currencyOptions = $observer->getEvent()->getCurrencyOptions();
        $currencyOptions->addData($this->getCurrencyOptions($baseCode));

        return $this;
    }

    /**
     * Get currency display options
     *
     * @param string $baseCode
     * @return array
     */
    protected function getCurrencyOptions($baseCode)
    {
        $currencyOptions = [];
        if ($baseCode) {
            $customCurrencySymbol = $this->symbolFactory->create()->getCurrencySymbol($baseCode);
            if ($customCurrencySymbol) {
                $currencyOptions[Currency::CURRENCY_OPTION_SYMBOL] = $customCurrencySymbol;
                $currencyOptions[Currency::CURRENCY_OPTION_DISPLAY] = \Magento\Framework\Currency::USE_SYMBOL;
            }
        }

        return $currencyOptions;
    }
}

---

UPDATED :

You loaded every time currency object using this below line and create object in construct. You should remove this below lines :

$this->currencyCode = $currencyFactory->create(); // Remove this class from construct
$currency = $this->currencyCode->load($currencyCode);
$currencySymbol = $currency->getCurrencySymbol();

and use this below line for get currency symbol :

$customCurrencySymbol = $this->symbolFactory->create()->getCurrencySymbol($baseCode);

Remove generated folder and clean cache.

Hope, It will helpful for you.