SQL Query To Eloquent Relationship

// category-product_variant pivot table
Schema::create('category_product_variant', function (Blueprint $table) {
    $table->id();
    $table->foreignId('category_id')->constrained('categories');
    $table->foreignId('product_variant_id')->constrained('product_variants');
    $table->timestamps();
    $table->softDeletes();

    // unique constraints
    $table->unique(['category_id', 'product_variant_id'], 'category_product_variant_unique');
});

// category model
class Category extends Model
{
    public function products()
    {
        // return $this->hasMany(Product::class); <-- only gets products directly connected to the category

        // WHAT TO DO HERE
    }
}

// raw sql
select * from `products`
where `category_id` = 6
   or exists (
       select * from `product_variants`
       where `products`.`id` = `product_variants`.`product_id`
         and exists (
             select * from `categories`
             inner join `category_product_variant`
             on `categories`.`id` = `category_product_variant`.`category_id`
             where `product_variants`.`id` = `category_product_variant`.`product_variant_id`
               and `categories`.`id` = 6
               and `categories`.`deleted_at` is null
         )
       and `product_variants`.`deleted_at` is null
   )
and `products`.`deleted_at` is null

i have the following models: Product, ProductVariant and Category. a Product belongs to a Category via category_id, it also has many ProductVariants via product_id on the product_variants table. a ProductVariant may have categories attached to it stored in a m2m pivot table category_product_variant. the raw sql above produces the desired results or data. i want to get all products in a category even if it's only the variant that's associated with it. for example a book. the book has the books category and one of its variants is connected to the audiobooks category. i want the book to show when i look for products in the audiobooks category.

public function products()
{
    $categoryId = $this->id;

    return Product::where('category_id', $categoryId)
        ->orWhere(function ($query) use ($categoryId) {
            $query->whereExists(function ($subquery) use ($categoryId) {
                $subquery->select(DB::raw(1))
                    ->from('product_variants')
                    ->whereColumn('products.id', 'product_variants.product_id')
                    ->whereExists(function ($subquery2) use ($categoryId) {
                        $subquery2->select(DB::raw(1))
                            ->from('categories')
                            ->join('category_product_variant', 'categories.id', '=', 'category_product_variant.category_id')
                            ->whereColumn('product_variants.id', 'category_product_variant.product_variant_id')
                            ->where('categories.id', $categoryId)
                            ->whereNull('categories.deleted_at');
                    })
                    ->whereNull('product_variants.deleted_at');
            });
        })
        ->whereNull('products.deleted_at');
}

this returns this error: App\Models\Category::products must return a relationship instance.