I have a few ideas for this, although I will not be able to pursue them now.

Ideally, a re-vamped NestedScrollView would solve these issues (among others, theses are just a few of several that stand out):

• NestedScrollView.body will be disconnected if horizontal #102001
  • the new structure would not rely on the PrimaryScrollController, and perhaps introduce a public NestedScrollController that inherits the current existing concept of a NestedScrollCoodinator. This would be similar to widget specific controllers like TabController and PageController, and could grant long requested access to the inner workings.
• ScrollController.maxScrollExtent of NestedScrollView limited to header (outer scroll), doesn't allow for inner CustomScrollView scrolling #62363
  • mentioned above, accessing the inner working of the NestedScrollView, or even just accurate metrics can be difficult for users. Any metrics or scroll notifications should be a composite of both scroll views
• NestedScrollView does not support SliverAppBar floating and snapping simultaneously #59189
  • similar: Overscrolling outer scrollview / SliverAppBar.stretch does not work in NestedScrollView #54059
  • NestedScrollView should support all sliver features and functionality
• Document how NestedScrollView.physics/scrollDirection/reverse affect outer/inner scroll views #45619
  • how it is configured should be clearer. right now it can be a mixed bag of configs, creating a host of issues.
• NestedScrollView's use of PrimaryScrollController breaks when there are multiple inner ScrollPositions #40740
  • The PrimaryScrollController in the NestedScrollView here is applying the same scroll offset to every attached position, since the NestedScrollView applies scroll deltas to every attached position.

Lastly, it would be awesome if we could do this without breaking anyone. :)