Dear @franksvamp
Hope you come to a solution soon, i don't have time at present sorry to guide you through, but may set you in a possible direction. 🌝
I was thinking of a faster hack but might not be elegant or work out correctly with your design:
You create 3 identical single-column "Project thumbnail grids" (a,b&c):
A B C
1 1 1
2 2 2
3 3 3
4 4 4
5 5 5
6 6 6
7 7 7
8 8 8
9 9 9
And then using Custom CSS remove the necessary ones to create the effect:
https://css-tricks.com/useful-nth-child-recipies/
A - B - C
1 - 4 - 7
2 - 5 - 8
3 - 6 - 9
Therefore:
Off column A was removed thumbnails 4,5,6,7,8,9
Off Column B was removed thumbnails 1,2,3 & 7,8,9
Off Column C was removed thumbnails 1,2,3,4,5,6
Best wishes
Richard