Laravel Eloquent fetch categories with top enrolled courses

I have these models:

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Category extends Model
{

   public function parentCategory()
   {
      return $this->belongsTo(self::class, 'parent_id');
   }

   public function subCategories()
   {
      return $this->hasMany(self::class, 'parent_id');
   }

   public function courses()
   {
      return $this->hasMany(Course::class);
   }

public function SubcategoryCourses()
{
    return $this->hasManyThrough(Course::class, self::class, 'parent_id', 'category_id', 'id');
}
...

and my Course model

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Course extends Model
{

   public function category()
   {
      return $this->belongsTo(Category::class);
   }

  public function students()
  {
     return $this->belongsToMany(User::class, 'enrollments', 'course_id', 'user_id')->withTimestamps();
  }

}
...

I would like to get the most popular parent categories, ie parent categories ordered by the number of students enrolled in subcategories. I know it is easy to get the subcategories ordered by count of student, but what I would like is actually get the parent categories ordered by the number of students. Eloquent or Query builder, doesn't matter to me.

I am on Laravel 5.5 and MySQL btw

via Chebli Mohamed