surely there would have been at least one star between it and Earth. Or am I (as a non astronomer) underestimating how much space there is in space?

Yes, you might be underestimating the emptiness of space, but it’s also a case of selection bias. The only stuff we see is stuff that isn’t blocked by something closer. There are black holes out there with a star between us and it, but we don’t know about them because we can’t see them.

A somewhat related fun fact is that we can’t see very far in the plane of our galaxy because our view ends up being blocked by stars and dust. So if you plot the position of the galaxies that we know about, the structure looks like two cones.

The way it is usually verbally described however I would have thought a black hole is in the center of a sphere so that the image should just look like a circle (no hole)

The system of around a black hole (or any other object in the universe) has an angular momentum that is just the sum of the angular momenta of all the parts that make it up. If it’s dense enough that collisions happen frequently (or given enough time), the collisions make it so that all of the constituent parts end up with angular momentum in about the same direction. This is why there’s a disc and not a sphere. Anything with a different angular momentum ends up crossing through the disc, colliding with the stuff in the disc.

And if it really is donut shaped then if it rotates 90⁰ we would see no hole just a thick line of the stuff circling it and yet we never see images like that.

We wouldn’t, actually. Because of the light-bending effects of a black hole, you see the opposite side of the disc over/under the black hole.