Guys! I developed on flutter_unity Plugin for Flutter Web and we achieved a very good state of stability.
But these Plugins were made Open Source by non-unity developers!
So it‘s in our hand to create a Plugin for UE4 or UE5!
It just should be possible to export from UE4/5 an android bundle (not a self running app) and for iOS a UE4/5 Framework library to use it. For Web it can be a self running app because its embedded as a WebGL App.
So we just need some smart minds to START DOING IT!