src/Page/CheckoutPageController.php
public function getViewer($action)
    {
        if (CheckoutPage::config()->first_step && $action == 'index') {
            $action = CheckoutPage::config()->first_step;
        }
        return parent::getViewer($action);
    }
public function getViewer($action)
    {
        if (CheckoutPage::config()->first_step && $action == 'index') {
            $action = CheckoutPage::config()->first_step;
        }
        return parent::getViewer($action);
    }
Override viewer to get correct template for first step

{@inheritDoc}
@see \SilverStripe\CMS\Controllers\ContentController::getViewer()