I have assassins in my tanks as well. They have gradually eaten most of my other snails down to a very manageable population; the assassins themselves are seen only rarely, often going completely undetected for months at a time. I tend to forget about them completely until I occasionally remove some of the accumulated empty shells of their victims.
If this were my tank I would remove about half of that sand. This can be done by simply siphoning some out every time you do a water change. No need to cause a sudden large disturbance, just gradually remove some each time until you get to the level you like.