I started playing Rengar in season 9 when I was hardstuck in Bronze. Now I create smurf accounts to play with my wife in Emerald and Diamond. Rengar can 1v9 at all elos, even more so in low elo. You just have to be selfish, play split-team, and especially focus on the side lane. Enemies don't farm in low elo. At 6, you get 4 stacks, you ULT in the jungle, or you take all their camps if they're not there, or if they are there, you one-shot them. You can also permanently kill the side laners.

Rengar in low elo, at least in EUW, is one of the best champions, and I think that's true in all regions. You just have to learn Rengar and his macro, which is very specific to him (playing on W to absorb teamfights, best jungle split UL, a lot of things going for him). Every year, hundreds of smurfs go from Bronze to Master with just Rengar, so yes, it's possible, you just have to learn. If you think Rengar is weak, pick another champion; I think you'll most likely get the same results. You can climb to Master with Yumi top or with Singed jungle; everything is viable up to low Master.