> Unfortunately, this conflicts with the (common, good) advice to make as many variables final / const as possible, since it separates the declaration of a variable and its assignment.

In Java you can separate the declaration and assignment of a final variable as long as every branch provably assigns to the variable. For example:

  final int x;
  if(someCondition) {
      final int a = 1;
      final int b = 2;
      x = a + b;
  } else {
      x = 0;
  }