There is a real progression and you have identified the gap correctly. The principle that fills it is use one leg, and change the leverage - because you cannot add plates, so you have to make the same bodyweight harder.
The ladder, roughly:
- Split squat, both feet on the floor, rear foot for balance only. Immediately harder than a two-leg squat because one leg is doing most of the work.
- Rear foot elevated split squat. Back foot on a chair or a bench. This is the single most valuable movement on the list and most people can spend months here productively.
- Step-ups to a high box, slow, no pushing off the bottom foot.
- Assisted pistol, holding a doorframe, or to a chair you touch and stand up from. This is the missing rung you could not find. Lower the chair over time.
- Full pistol.
And separately, because it is a different muscle group entirely: hamstring work. Nordic curl progressions, or single-leg hip hinges. Squat patterns barely touch the back of the leg, which is a large part of why bodyweight leg training gets a bad reputation.
If fifty squats are easy, you belong at step two, not step one. Elevate the rear foot tomorrow and you will find the difficulty immediately.