Wait WHAT
I didn't know there was a Difference filter, that would make my life significantly easier!
I'll check it out rn
Ok, after some trial and error, i even found a combination of filters, that allow Krita automatically to create those masks for me - as in, i just need to drag in a background image and the image, i want to get the difference as an overlay, and it now automatically cuts out this area (thanks to the difference filter as fist step), then a recoloring filter recolors those differences to a black and white mask, then a clone of the image from which i want the difference is multiplied with that black and white mask, to get the color of this area, and the rest stays pure black. And the last step was just a filter to convert a defined color (black in my case) to transparency. or maybe change the order, so that you already convert the black and white mask to transparent and white, then multiply that with the color - that way even black in the difference will be kept and not accidently converted to transparency.
Now its just dragging in images and saving the result as a new overlay for background images.
I have tested it with some of your images, all in all was able to reduce the size by half for those, granted that i had no clean backgorund and needed to cover old chars too.
Your biggest problem is, that many images do indeed have slight camera movements, which ruin this, but there are still enough images, where just the chars poses change (and some anti aliasing "errors"), for which this still can make a significant difference.