I would just fill in that huge gap with some pretty round rocks (I don't know how they would damage the pool?) or some mulch. Are you planning on leaving the pool up year round or do you have to take it down in the fall? That would help determine what kind of filler you put around it. You already have the trench around the pool and it looks stable so now I think you just need something attractive to fill that gap. I think only sharp rocks would put your liner in jeopardy.