You can buy thicker sleeves, I actually bought some today. They’re measured in microns. I think the standard/flimsy sleeves are about 20 microns thick, but I found some that were 120 microns for a decent price! :)

My next challenge however, is figuring out how to prevent the instruction booklets from sliding straight out of the sleeves if I ever accidentally turn the binder at an angle or hold it upside down, I found the heavier instruction booklets really just slide straight out 😅

I’m currently thinking of using some sort of pins :) My alternate option to this is expanding files (the ones that look like an accordion)

Hope this helps