Free payments with custom Offsite Gateway

Describe your bug or feature request.

The new interface for handling free orders → is great, but our sole offsite custom payment gateway does not yet implement this new interface.
Reference: commerce_touchnet_upay.

After exploring multiple approaches, I'm trying to determine the recommended solution.
Our options seem to be:

• Assign the order to the manual payment gateway (but that gateway does not implement SupportsZeroBalanceOrderInterface, which enforces SupportsStoredPaymentMethodsInterface as I would have expected... follow-up issue?).
• Implement SupportsZeroBalanceOrderInterface along with SupportsStoredPaymentMethodsInterface in commerce_touchnet_upay, upgrade to 3.x, and resolve the issue properly.

Context (Commerce 2.x):

In 2.x, we encountered an issue where a product with a price of $0 triggers an error in finalizeCart().
This happens because the payment gateway is null when processing a zero-value order.
Interestingly, this issue does not occur when a product is discounted to $0—only when its original price is $0, such as for a free event product.

As a workaround, we manually assign a payment gateway if none is set for a zero-value order:

/**
 * Implements hook_commerce_order_update().
 */
function MODULE_commerce_commerce_order_update(OrderInterface $order) {
  // Ensure the order has an assigned payment gateway for zero-value orders.
  if ($order->getTotalPrice()->isZero() && $order->get('payment_gateway')->isEmpty()) {
    $payment_gateway = PaymentGateway::load('upay');
    if ($payment_gateway) {
      $order->set('payment_gateway', $payment_gateway->id());
      $order->save();
    }
  }
}

Steps to reproduce (clean install):

1. Enable an offsite payment gateway.
2. Create a product with a price of $0.
3. Proceed to checkout.
4. Encounter errors:
   • In 3.x: There are no payment gateways available for this order. (when the interface is not implemented).
   • In 2.x: Error: Call to a member function getPlugin() on null in Drupal\commerce_cart\CartProvider->finalizeCart() (line 127 of commerce/modules/cart/src/CartProvider.php).

Request for Guidance:

Should we:

• Assign free orders to the manual payment gateway despite it not implementing SupportsZeroBalanceOrderInterface?
• Implement SupportsZeroBalanceOrderInterface and SupportsStoredPaymentMethodsInterface in commerce_touchnet_upay?
• Handle this scenario differently?

Any recommendations would be appreciated! I made this a support request because it's been reported in various other ways including the parent related issues, and it's more of a question on the correct approach to take from maintainers