I try to add more KO functions to the Magento_Checkout/template/summary.html

I have an own extension "XXX_Checkout"

For this i do the follow parts:

1. Create Mixin

> /app/code/XXX/Checkout/view/frontend/requirejs-config.js

        var config = {
        map: {
            '*': {
                'Magento_Checkout/js/view/shipping': 'XXX_Checkout/js/view/shipping',
                'Magento_Checkout/js/view/payment': 'XXX_Checkout/js/view/payment',
                'Magento_Checkout/js/model/shipping-save-processor/default': 'XXX_Checkout/js/model/shipping-save-processor/default',
                'Magento_Checkout/template/summary.html': 'XXX_Checkout/template/summary.html'
            }
        },
        config: {
            mixins: {
                'Magento_Checkout/js/action/set-payment-information': {
                    'XXX_Checkout/js/order/set-payment-information-mixin': true
                },
                'Magento_Checkout/js/view/summary': {
                    'XXX_Checkout/js/view/summary-mixin': true
                }
            }
        }
    };

For this i place the original summary.html in my Extension and place some additional code:

> /app/code/XXX/Checkout/view/frontend/web/template/summary.html

    <div class="opc-block-summary" data-bind="blockLoader: isLoading">
        <span data-bind="i18n: 'Order Summary'" class="title"></span>
        <!-- ko foreach: elems() -->
        <!-- ko template: getTemplate() --><!-- /ko -->
        <!-- /ko -->
    </div>
    
    <!-- ko if: (isVisiblePaymentButton()) -->
    <div class="actions-toolbar-trigger" id="place-order-trigger-wrapper">
        <button type="button" class="button action primary" id="place-order-trigger">
            <span><!-- ko i18n: 'Place Order'--><!-- /ko --></span>
        </button>
    </div>
    <!-- /ko -->

Now i add the summary-mixin.js

> /app/code/Team23/Checkout/view/frontend/web/js/view/summary-mixin.js

    define([
        'jquery',
        'jquery/ui',
        'ko',
        'Magento_Checkout/js/model/step-navigator'
    ], function($, ui, ko, stepNavigator){
        'use strict';
        var mixin = {
            isVisibleShippingButton: function () {
                return !stepNavigator.getActiveItemIndex();
            },
    
            isVisiblePaymentButton: function () {
                return stepNavigator.getActiveItemIndex();
            },
    
            initialize: function () {
                var self = this;
    
                $(function() {
                    $('body').on('click', '#place-order-trigger', function () {
                        $('.payment-method._active').find('.action.primary.checkout').trigger('click');
                    });
                });
    
                this._super();
            }
        };
    
        return function (target) {
            return target.extend(mixin);
        };
    });

After build and cache clean i get this error in my console:

    knockout.js:3012 Uncaught ReferenceError: Unable to process binding "if: function(){return (isVisiblePaymentButton()) }"
    Message: isVisiblePaymentButton is not defined
        at if (eval at createBindingsStringEvaluator (knockout.js:2624), 

So i think, the mixin is not working.

What is wrong?

Best regards
Andre