In the past, with analogue consoles, you could always do anything you wanted. But even with them there was no solution to route a subgroup directly into another subgroup. And it was precisely for such applications that the matrix was invented.
The nice thing about a matrix is that you can use it as a collecting basin for all kinds of signals. No matter if they are subgroups, or aux paths, or since some time also single inputs.
The simple solution to your problem could be to use a matrix instead of a “master subgroup”.