Hi Buster,
My workflow is to export video from After Effects as numbered frames, load the frames into Adobe Animate then select all frames in the library panel, right click and select create sprite sheet - create without spaces. I then import sprite sheet into Photoshop and crop any empty space so I have the right size for the rows and columns of frames, I usually sharpen and boost the colours also. Then I will run the image through ImageOptim to bring the size down to something acceptable to Google DoubleClick Studio.
Hope this helps.