I see your point ...
and I really had to think some time whether your idea was so obvious that it is a pity that the function does not already work like it.
And then I remembered this thread:
The suggested format string to remove duplicates from genre was originally used for a different field.
And then I rememberd that the suggested filter expression to find the number of genres was originally used to determine the folder depth ...
So it is not always that the format strings are used only in 1 context.
Which means that we end up with the unavoidable request for an "option". Or perhaps a different sorting (first those for the current field, then all the others).
I see that also.
As you described: most times you use the particular expressions for just one field.
But then it could happen that you look for that one string but you don't remember for which field you used it ... And then it would be nice to see all strings again.
Or you have such a list of user-defined fields like in the linked thread (genre1, genre2, genre3) and even if you want to apply almost identical functions to each of these fields, you either have to type them in completely or copy&paste them.
So, my fear, only an option to see the filtered or the complete list right next to the string selection would be the only way to make everybody happy.