Thank you again for your detailed response. I'm having a hard time wrapping my head around using the grid system for this layout without using flexbox. I have only used flexbox in my limited time trying to develop things (I'm a designer, learning development on my own).
From my understanding the stacking works much better using grid, but I'm not understanding how to achieve the 3 columns of images per row, or if I even need to use rows anymore. Grid seems to spread things out automatically without the use of rows. Again, sorry for my ignorance, I'm learning!
Once I can understand that, I think I can stack these at the beginning, and then work on the animation per your helpful codepens and thread.