The diagnostic you are already using without naming it: if you can do fifty of something, it is not strength work.
Strength adaptation wants a load you can move for a low number of hard repetitions. Somewhere in the region of five to twelve reps where the last two or three are genuinely difficult. Above about twenty you have moved into endurance, and that is a real quality but it is not the one you are trying to change.
So the job is to find a variation where eight is hard. That is the whole selection criterion, and it applies at every stage.
Ways to make a leg movement harder without weights, in rough order of usefulness:
- Fewer limbs. One leg instead of two. The biggest single jump available.
- More range. Elevate the front foot in a split squat, or the whole body in a step-up.
- Slower. A five second lowering phase changes a movement completely.
- Pauses. Three seconds at the bottom removes any bounce and exposes the position.
- Less leverage. Moving the load further from the joint.
Combine two of those and almost anything becomes hard for eight. A rear-foot-elevated split squat with a three second lower and a pause at the bottom is a serious exercise for most people, and it needs a chair.