Been putting this off knowing it has been cancelled, but I really liked Dark and curiosity wins.
...done.
About that ending... some light spoilage:
The ending has finality. I was expecting some kind of cliff hanger, like they did halfway through Dark, but this feels OK as a stopping point. You have plenty of hooks you could expand on, but there's really no need. And now that the surprise is unveiled, it kind of takes away the premise.
I give the Dark writers credit, they could probably keep building on this, but there's no need to.