Using bottom navigation with individual Navigators but single AppBar

Question

I am trying to create an app that has the following basic setup for navigation:

tab1 -> subpage
tab2
tab3

with a bottom navigation bar for switching between the tabs and each tab having its own navigation stack. Furthermore I would like to add an AppBar with a drawer (as endDrawer) and the back button as leading widget in the AppBar.
The AppBar should be centrally managed so I put it in the main page of the app for maintenance purposes instead of having it on every page that is pushed. The drawer should also be updated later on when the user is logging in or out so it encouraged me more to place the AppBar only in the main page.

Now the issue is that I would like to update the AppBar to contain the back button when the app navigated to the subpage of tab1. Since the AppBar is in the parent I thought of RouteObservers as a possible solution by registering a RouteObserver on each NavigationState.
Yet I cannot get it to work because when I would like to subscribe on the RouteObserver in the HomePage in the didChangeDependencies() the currentState of the NavigatorState does not exist. Also I tried subscribing in the build method after the Scaffold with a flag for subscribing only once, but this didn't work either.

I created a github project with the basics of my current implementation:
https://github.com/S-Braeutigam/bottom-navbar-single-appbar

To get to the current state of code I used the following links:
https://codewithandrea.com/articles/multiple-navigators-bottom-navigation-bar/
https://api.flutter.dev/flutter/widgets/RouteObserver-class.html
Flutter 2.0 appbar back button disappeared if contains endDrawer (The solution gets close to what I would like to implement, but the AppBar should always be visible, not only on the sub page)

My main questions are:

1. Are RouteObservers the correct approach on how to track the navigation to subpages of tabs and display the back button based on it?
2. At what lifecycle hook in the main page should or could the subscription on the currentState of each NavigatorState be done? Wasn't able to find any where they are not null.