add_rewrite_rule works in functions.php but not in plugin

Hi folks,
I’m new to WordPress particularly the development side of it, which I’m trying to get to grips with. I cannot seem to resolve an issue with code that works in functions.php but fails to work in a site-specific plugin.

The code in functions.php:

        add_action('init', function () {
          add_rewrite_tag('%make_id%', '([0-9]+)');
          add_rewrite_rule('api/v1/makes/([0-9]+)/?', 'index.php?make_id=$matches[1]', 'top');
        });

        // template redirect
        add_action('template_redirect', function () {
          global $wp_query;

          $make_id = $wp_query->get('make_id');

          if (!empty($make_id)) {
            wp_send_json_success([
              'message' => 'hello world'
            ]);

          }
        });

The above works fine after I save changes in Permalink Settings and navigate to the url https://example.com/api/v1/makes/1/

However, when the same code is in the plugin I just get a 404 page.

Plugin code:

<?php
  /*
   * Plugin Name: First Plugin
   * Description: My first WordPress plugin that does nothing.
   * Version: 1.0.0
   * Author: Malcolm Dixon
   * License: GPLv2
   */

  // Exit if accessed directly.
  if (!defined('ABSPATH')) exit;

  /* CLASS */
  if (!class_exists('First_Plugin')) {
    class First_Plugin {

      const API_BASE_URL = 'api/v1/';

      function __construct() {
        /*
        * activate, deactivate, uninstall hooks
        */

        register_activation_hook(__FILE__, [$this, 'activate']);
        register_deactivation_hook(__FILE__, [$this, 'deactivate']);
        register_uninstall_hook(__FILE__, [$this, 'uninstall']);
      }
  
      /* METHODS */
        
      /*
      * Activate the plugin
      */
      function activate() { 
        /* add rewrite tag and rule for custom endpoint */
        add_action('init', function () {
          add_rewrite_tag('%make_id%', '([0-9]+)');
          add_rewrite_rule('api/v1/makes/([0-9]+)/?', 'index.php?make_id=$matches[1]', 'top');
        });

        // template redirect
        add_action('template_redirect', function () {
          global $wp_query;

          $make_id = $wp_query->get('make_id');

          if (!empty($make_id)) {
            wp_send_json_success([
              'message' => 'hello world'
            ]);

          }
        });
        
        // Recreate rewrite rules
        flush_rewrite_rules();
      }

      /*
      * Deactivate the plugin
      */
      function deactivate() {
        //Nothing here yet
      }

      /*
      * Uninstall the plugin
      */
      function uninstall() {
        // Nothing here yet
      }
    }

    /* MAIN */
    function first_plugin() {
      global $fp;
      
      // Instantiate only once.
      if(!isset($fp)) {
        $fp = new First_Plugin();
      }
      return $fp;
    }
  
    // Instantiate.
    first_plugin();
    
  }
?>

I’m probably missing something obvious being new to PHP I wouldn’t be surprised if the code is not even being called.

Any advice appreciated.