We definitely made progress on this.
This filter works great for 5 of the 8 WPSP grids I’m using. By this I mean, if I do not specify an image size in WPSP, then the image is served with the <picture> tag as expected.
However, I still have 3 WPSP grids do not use the default image size.
This filter only sets the default size, and so anytime I set the size in WPSP it does not use the <picture> tag.