What confuses me about your sandbox example: where do you define the container for the Modeler and the Properties Panel? It seems you try to attach them at a later point in time, but I don’t see the exact place.
Btw.: there are late discussions in regards to integrating the properties panel into an Angular application: Error while using bpmn-js-properties-panel and camunda-bpmn-moddle in angular10 application. Did you already read through those threads?