Unable to run drush via php's `exec` command

Question

I'm creating an admin page for my module that will allow an admin to trigger scripts from within the module configurations.

So far, I've created these routes:

my_module.scripts.home:
  path: "/admin/config/development/my_module/scripts"
  defaults:
    _title: "Scripts"
    _controller: '\Drupal\my_module\Controller\ScriptsController::home'
  requirements:
    _permission: "administer site configuration"
my_module.scripts.run:
  path: "/admin/config/development/my_module/scripts/run/{script}"
  defaults:
    _title: "Run Script"
'\Drupal\ytp_integrations\Controller\ScriptsController::runScript'
    name: "none"
  requirements:
    _permission: "administer site configuration"

And then my Scripts Controller looks like so:

<?php

namespace Drupal\my_module\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\Yaml\Yaml;
use Drupal\Core\Url;

/**
 * Defines Configurations Controller class.
 * 
 * This class controller i
 */
class ScriptsController extends ControllerBase {

  /** 
   * Home
   */
  function home() {
    $build['table'] = [
      '#type' => 'table',
      '#header' => [
        'Title',
        'Description',
        'Path',
        'Status',
      ],
    ];

    // Retrieve all scripts
    $scripts_config_file = file_get_contents(drupal_get_path('module', 'my_module') . '/my_module.scripts.yml');
    $script_config = Yaml::parse($scripts_config_file);

    // Create Table Rows and add to table
    $rows = [];
    foreach($script_config as $key => $script) {
      $rows[] = [
        'title' => $script['title'],
        'description' => $script['description'],
        'path' => $script['path'],
        'operations' => [
          'data' => [
            '#type' => 'link',
            '#title' => 'Run',
            '#url' => Url::fromRoute('my_module.scripts.run', ['script' => $key])
          ],
        ],
      ];  
    }
    $build['table']['#rows'] = $rows;

    return [
      '#type' => '#markup',
      '#markup' => render($build),
    ];
  }

  /** 
   * Run Script
   * 
   * 
   */
  function runScript($script) {
    // Retrieve the script
    $module_path = drupal_get_path('module', 'my_module');
    $script_config = Yaml::parse(file_get_contents("$module_path/my_module.scripts.yml"));
    $script_data = $script_config[$script];

    $script_path = $module_path . '/' . $script_data['path'];

    $execute = exec("drush scr $script_path > /dev/null &");

    \Drupal::logger('ytp')->notice('<pre><code>' . print_r($execute, TRUE) . '</code></pre>');

    return new RedirectResponse(Url::fromRoute('ytp_integrations.scripts.home')->toString());
  }
}

Structurally, this seems to all work, but when I print out the execute variable after running the drush command via exec, I get nothing and the code in my script isn't executed. I changed the command to pwd and it printed out the path like expected, but exec doesn't seem to know what drush is...

I would use drush_invoke_process, but I would like my drush command to be executed in the background. I'm expecting to get a pid back so that I am able to determine which scripts should have their run button disabled if the pid is running in the background.

Any reason why exec doesn't know what drush is or how I could kick off my drush script in the background running via Drush?