This is all great info! Lets all try different things using the experience documented here.
We will find out if smooth concrete with channels sawed in it work
We can try rough concrete like concrete block with channels saw in it
We can try different types of plastic/acrylic and different colors.

I will continue to try the grooves in the tread in my tires (obviously I only can choose black in color)
I will make some more plastic disk or square in stacks.

I also plan to pull my pallet structure out and use a saw to add 3/16 to 1/4" grooves on the top side. It sounds like stacking 1/2" sheets of concrete would be wise since they like horizontal grooves but in various heights from bottom of the pond to top so your vertical concrete will give both horizontal grooves but will allow them to present at various depths.