If that rack is the kind that the two legs hinge to make it fit more bikes, that is not a very stiff rack. If it is the one where the clamp just clamps onto the seatstays and depends on friction to keep it from sliding down, I replaced mine years ago. With the old sidepull brakes I had on one bike, when that clamp slid down the seatstays to the brake, it pushed one brake pad on the rim. And of course no tools in my pocket to fix that.

I think you can do better, those racks were not really designed for panniers.