I had very good results with a string trimmer and wet leaves. They shredded very, very well.
I put leaves in a plastic trash can, donned safety goggles and worked the trimmer up and down a few times. The end result looked more like stiff, cooked oatmeal than the dry oatmeal flake look. I saved some "pulp" & dried it to see how it looked. The pieces were tiny, little flakes.
The end product then got mixed in my garden dirt with a shovel. If it isn't totally disintegrated by spring, it will serve as air entraining add-mix to our heavy, clay garden.
Dry Leaves-
When the leaves were dry & the can standing vertically, I ended up wearing more pieces than stayed in the can (but it was fun). Propping the can so it was at about a 45 degree angle to the ground (opening up) helped keep the tiny, dry pieces in the can.
Science Project-
Next spring, when the 'wasters' throw away string trimmers & trash cans that dare to be dirty, I think I'll trash pick one of each and experiment with making a thing similar to the commercial string trimmer leaf shredders. (Example- SunJoe SDJ616) It won't be time-efficient to build, but why not re-use already existing plastic instead of causing more to be made by buying a new tool?
PS: Always remember to wear safety eye protection with any rotating or striking tool (and more...).
Have Fun Leaf Wrecking!
Paul