Spegel does not only mirror Docker Hub, and works a lot differently than the alternatives you suggested. Instead of being yet another failure point closer to your production environment, it runs a distributed stateless registry inside of your Kubernetes cluster. By piggy backing off of Containerds image store it will distribute already pulled images inside of the cluster.
I'll be honest and say I hadn't heard of Spegel before, and just read the landing page which says "Speed up container pulls and minimize downtime with a stateless peer-to-peer OCI registry mirror for efficient image distribution", so it isn't exactly clear you can use it for more things than container images.
Spegel itself does not manage state as a normal registry would. Maybe ephemeral would be a better word to describe it. A normal registry would require some stateful storage solution along with a database to store image that clients push to it. Spegel exploits the fact that images used by containers will be stored on disk by Containerd for its benefit. Any image currently being used by a pod in a cluster will be available for all other nodes in the cluster to pull.
Gotcha. That's definitely an important point and seems difficult to communicate in a single word or quick blurb. I can see why you went with stateless. It's just a little confusing in this context (for me at least).
