Thank you Shinichiro! Your response is much appreciated.
I had a similar need a few weeks back and took a different approach that works just as well or maybe even better.
I created a standard highlight table in Tableau.
But then I added a few bar charts for the grand totals along the top and side.
These do the trick to show grand totals.
I did a bunch of research on this topic and compiled all of my notes into an 800+ word guide...
The guide is broken up into 5 easy to follow steps so you can avoid rookie mistakes and save time.
Check out the full Tableau highlight table tutorial here: https://newprediction.com/tableau-highlight-tables/
I have a 7-page PDF summary and 10-minute video walkthrough included too.