Are there biological ratchet cycles that are small in number?
First I would in return ask why the actin-myosin coupling should not qualify - the many repetitions of this contractile unit never contract at exactly the same time, and their combined contractile force certainly accumulates in a ratchet-like fashion across the microsecond time scale (with other units contracting before previous units have relaxed). Just because it's a lot of them doesn't make it less ratchet-like :)

With this restriction though, and looking for a "macro-scale" example, I am not aware of anything fitting your description. If I think about it as follows, it also seems intuitive that this wouldn't evolve.

A ratchet as you describe, let's say able to store up x contractions of a muscle for instantaneous release, can also be thought of as a series of x muscle contractions coupled in series across time. It would be equivalent in output to coupling x muscle contractions across space and executing all of them simultaneously. In other words, growing x times the muscles achieves the same benefit.

When it comes to the probability of evolving, what's more likely: an entirely new mechanism to store mechanical energy across time and allow building up, or a simple duplication of the same tissue? The currently existing muscles answer this question: duplication. This can be seen in the different structures that muscles have across living species (muscles in animals, similar contractile mechanisms elsewhere).

The smallest contractile unit in muscles are myosin heads coupled to actin filaments. Duplicate this unit (repeatedly) in two dimensions of space to obtain a myofibril. Multiply the myofibril many times and you reach a single muscle fibre. In fruit fly larvae, this is where it stops - each muscle consists of a single fibre. By the time you're at the adult fly though, the muscles that power the wings consist of a handful of packed parallel fibres. As you go down other routes along the evolutionary tree towards the mammals, you end up at muscles that consist of masses of fibres packed into fascicles, of which many are packed into an individiual muscle. As described above, at each level you have a "many-ratchet", because fibres and fascicles don't contract at exactly the same time everywhere.

So yes, you could evolve a ratchet. Or you could just evolve a bigger muscle. If many animals happen to stop evolving more muscle at around the same absolute potential jump height, that to me suggests that this is a universally beneficial jump height to evolve for terrestrial animals regardless of body size.