Magento – How to override vendor module in magento 2 via code

I couldn't reproduce your error, but I got an other one.

Then I did some minor adjustments to your code and it worked.
First drop the InstallSchema.php file. It does nothing so it is useless.

Then make sure that your plugin file is in Amaj/CorePlugin/Model/Currency/Plugin.php and it looks like this:

<?php

namespace Amaj\CorePlugin\Model\Currency;

class Plugin
{
    protected $logger;

    public function __construct(\Psr\Log\LoggerInterface $logger) {
        $this->logger = $logger;
    }

    /**
     * @param $subject
     * @param \Closure $proceed
     * @param $price
     * @param array $options
     * @return string
     */
    public function aroundFormatTxt(
        $subject,
        \Closure $proceed,
        $price,
        $options = []
    )
    {
        $this->logger->info('Amaj aroundFormatTxt');
        return '2';
    }

}

you don't need parent::__construct(); in the constructor because there is no parent class.
And I replaced $logger->info with $this->logger->info because there is no $logger variable.

You cannot override that class.
Magento\Email\Model\AbstractTemplate is an abstract class and it never gets instantiated.
Your code is correct but you don't see your method being called because your class is never instantiated.
preferences work only for classes that get instantiated.
You can do one of the following.
- Create an around plugin for the method. Plugins work with abstract classes because they will be called also in the child classes of the abstract ones.
- Rewrite the method in every child class of the AbstractTemplate class. I don't really recommend this since it's painful and you can easily miss something.