Interesting. So, you are creating a Monogram placeholder for 4 core surname lines in your Tree by repurposing the Family Nickname. (I did not write “Initial” because in computer usages, this could be mistaken for a form of “initialization”, or setting of a default form/value,) I had mistaken the objective as populating the Tree with a commonly used Family Nickname.
I have not tried it, but I wonder if a customized Name Format (created in the Display tab of Preferences) of the “Display As:” might be an alternative approach. And one that could be applied en masse for a Surname.
It would require some experimenting. Because I suspect you would have to create 4 display format templates (since there isn’t an automatic Monogramming format). And I am not confident all Report/Chart features will apply the “Display As”. (The individual contributing programmers had varying familiarity with the Gramps name options… some may not have even realized that preformatting exists.)