I can confirm, that both solutions are working, but with a difference.
When I use the offline method, the website is added to the home screen, but not as a separate app. The chrome logo is in the bottom right corner and when I long press on the app icon and select “more information”, it just shows me the chrome app:
When I use port 443 instead of 8123 and add it to home screen, chrome actually created an app. And when I long click on the app icon and select “more information”, the app has the name “Assistant”:
So I’m assuming, that the second one (with changing the port) is actually the correct one.
Thanks a lot guys!