I agree, I think sorting them is the most reasonable way to go. If you wanted to get nuts you could try using a layer of sand or rice on a tray and nesting the slate to equal heights, but I don’t know if it’d be worth it.
Natural slate works fine but just in case you wanted a bit more contrast, a very light coat of mineral oil works well. You can read about it here.
I personally prefer a more natural look, but the shellac makes for even greater contrast.