AbstractType.php

<?php

namespace CoenJacobs\StefExample\PostTypes;

abstract class AbstractType {
  public function setup() {
    // possibly some setup logic here?
  }
  
  public function register() {
    // full logic of registering the post type
    register_post_type( ... );
  }
}

Activity.php

<?php

namespace CoenJacobs\StefExample\PostTypes;

class Activity extends AbstractType {
  
}

Manager.php

<?php

namespace CoenJacobs\StefExample\PostTypes;

class Manager {
  protected $types = [];
  
  public function setup() {
    foreach( $this->types as $type ) {
      $type->setup();
    }
    
    add_action('init', [$this, 'registerTypes'];
  }
               
  public function addType(AbstractType $type) {
    array_push($this->types, $type);
  }
               
  public function registerTypes() {
    foreach( $this->types as $type ) {
      $type->register();
    }
  }
}

Plugin.php

<?php

namespace CoenJacobs\StefExample;

use CoenJacobs\StefExample\PostTypes\Manager;

class Plugin {
  /** @var Manager */
  public $post_types;
  
  public function __construct() {
    $this->post_types = new Manager();
  }
  
  public function setup() {
    $this->post_types->setup();
  }
}

stef-example.php

<?php

/**
 * Plugin Name: Stef Example
 */

require('vendor/autoload.php');

add_action('plugins_loaded', function() {
  $plugin = new CoenJacobs\StefExample\Plugin();
  $plugin->setup();
});