When you put it back in place, I would have the power for the pump turned off, and fill the tank with water. That will minimize the tendency to float.
Did you cut steel mesh when you cut into the concrete? It would have been good to have that bent out of the way during work, and bent back before putting the concrete into place. Maybe you could add a little steel.
I used fiber-filled cement mix when I did a sump.
I don't know which would be better for fill, but if it matters, gravel is probably better. Once your concrete is set, things should be good. Don't cement over the cleanout for your pit.