I bought the 20kg one recently after having had a couple of different ones and it’s easily the best one I’ve purchased.
I have done in the past, hill sprints/runs are the best way to mitigate against any joint damage. I found it to be a good way to progressive overload hill sprints, once you want to stop adding reps you can start adding weight.
There’s also incorporating them into bodyweight conditioning circuits, Jim Wendlers Walrus training is a fantastic example of this.
I used to run in all my kit in the Army, in boots or those stupid toe shoes that were popular back then, on pavement. In retrospect, I don’t think it did me any long-term favors (to your point about knees). I was a lot tougher back then, though.
Sprinting on forgiving surfaces is fine. I don’t recommend running. If I could go back in time, I’d undo all those miles I logged and replace them with something more forgiving. The pounding is hard on your knees long term.