Magento 2 : Custom Send Email - (Tested on Magento 2.2.6)

CustomMail.php

<?php 
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <phuong.le@menincode.com>
 * @copyright Copyright (c) 2020 Men In Code Ltd (https://www.menincode.com)
 */

namespace Namespace\MyModule\Model\SendMail;

use Magento\Framework\App\Area;
use Magento\Framework\Mail\Template\TransportBuilder;

class CustomMail implement CustomMailInterface
  
{
    /**
     * CustomMail constructor.
     * @param TransportBuilder $transportBuilder
     */
    public function __construct(
      TransportBuilder $transportBuilder
    ){
        $this->transportBuilder = $transportBuilder;
    }
  
    /**
     * @inheritDoc
     */
    public function send(TemplateVars $templateVars)
    {
        $this->prepareMail($templateVars);
        $this->transportBuilder->getTransport()->sendMessage();
    }
  
    /**
     * @param TemplateVars $templateVars
     */
    protected function prepareMail(TemplateVars $templateVars)
    {
        $vars = $templateVars->toArray();
        $store = $vars['store'];
        $templateId = $vars['template'];
        $this->transportBuilder->setTemplateIdentifier($templateId);
        $this->transportBuilder->setTemplateOptions(
            ['area' => Area::AREA_FRONTEND ,'store' => $store->getId()]
        );
        $this->transportBuilder->setTemplateVars($vars);
      
        $sender = "sender@myshop.com"
        $this->transportBuilder->setFrom(['email' => $sender, 'name' => 'Sales']);
      
        $address =['customer@somewhere.com','customer2@somewhere.com'];
        $this->transportBuilder->addTo(
            $address,
            $store->getWebsite()->getName()
        );
    }
}
  

CustomMailInterface.php

<?php 
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <phuong.le@menincode.com>
 * @copyright Copyright (c) 2020 Men In Code Ltd (https://www.menincode.com)
 */

namespace Namespace\MyModule\Model\SendMail;

use Namespace\MyModule\Model\SendMail\TemplateVars;

interface CustomMailInterface
  
{
    /**
     * @param TemplateVars $templateVars
     * @return void
     */
    public function send(TemplateVars $templateVars);
}

di.xml

<?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="Namespace\MyModule\Model\SendMail\CustomMailInterface" 
                  type="Namespace\MyModule\Model\SendMail\CustomMail" />
  
</config>

email-template.html

<!--@subject My Custom Email Subject @-->

{{template config_path="design/email/header_template"}}

<table>
  <tr>
    <td>
      <h1>{{var store.getFrontendName()}}</h1>
    </td>
  </tr>
  <tr>
    <td>
      {{var message}}
    </td>
  </tr>
    <tr>
    <td>
      Hello World , I can use my custom variable like that : {{var custom_var}}
    </td>
  </tr>
</table>

{{template config_path="design/email/footer_template"}}

email_templates.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Email:etc/email_templates.xsd">
  
    <template id="mycustom_email_template" label="Custom Email Template" 
              file="email-template.html" type="html" module="Namespace_MyModule" area="frontend"/>
</config>
 

SendMailHelper.php

<?php 
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <phuong.le@menincode.com>
 * @copyright Copyright (c) 2020 Men In Code Ltd (https://www.menincode.com)
 */

namespace Namespace\MyModule\Helper\Mail;

use Namespace\MyModule\Model\SendMail\CustomMailInterface;
use Namespace\MyModule\Model\SendMail\TemplateVarsFactory;

class SendMailHelper
{
  
   /**
    * SendMailHelper constructor.
    * @param CustomMailInterface $customMail
    * @param TemplateVarsFactory $templateVarsFactory
    */
    public function __construct(
        CustomMailInterface $customMail,
        TemplateVarsFactory $templateVarsFactory
    ) {
        $this->customMail = $adminMail;
        $this->templateVarsFactory = $templateVarsFactory;
    }

    /**
     * @param array $data
     */
    public function sendMail(array $data)
    {
        $this->customMail->send($this->templateVarsFactory->create(['data' => $data]));
    }

}

TemplateVars.php

<?php 
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <phuong.le@menincode.com>
 * @copyright Copyright (c) 2020 Men In Code Ltd (https://www.menincode.com)
 */

namespace Namespace\MyModule\Model\SendMail;

class TemplateVars
{
    /**
     * @var array
     */
    protected $data;

    /**
     * CustomMailVars constructor.
     * @param array $data
     */
    public function __construct(array $data)
    {
        $this->data = $data;
    }

    /**
     * @return array
     */
    public function toArray()
    {
        return $this->data;
    }
}

YourCustomClass.php

<?php 
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <phuong.le@menincode.com>
 * @copyright Copyright (c) 2020 Men In Code Ltd (https://www.menincode.com)
 */

namespace Namespace\MyModule\SomeWehere

use Namespace\MyModule\Helper\Mail\SendMailHelper;
use Magento\Store\Api\StoreRepositoryInterface;

class YourCustomClass 
{
  
    protected $customMail;
  
    /**
     * YourCustomClass constructor.
     * @param CustomMailInterface $customMail
     * @param StoreRepositoryInterface $storeRepository
     */
    public function __construct(
      SendMailHelper $sendMailHelper
      StoreRepositoryInterface $storeRepository
    )
    {
        $this->sendMailHelper = $sendMailHelper;
        $this->storeRepository = $storeRepository;
    }

     public function sendMail() 
     {
        $store_id = '2';
        $store = $this->storeRepository->getById($store_id);
        $data = [
                  'template' => 'mycustom_email_template'),
                  'store' => $store,
                  'custom_var' => 'put what you want',
                  'message'  => __("My custom email message)
                ];
        $this->sendMailHelper->send($data);    
     }
}