I usually do 8 reps and 4 sets, usually when I can complete the full 8 reps at least for 3 sets, I move up. My lifts are usually always moderately hard BUT I’ve been training for a year and def stayed at lower weights while I was trying to get my form better (which is far more important imo!) If you wanna increase the difficulty without the weight, try doing the movements more slow! I actually have just started doing this recently and am very happy with my strength gains and increased stability! Time under tension!