Hi - grooming this fluffy keyring I have, attempting a photorealistic asset match. One image shows the original photo, and then the other is the cleaned backplate with my render on top of it. What can I do to take this to a greater level of realism?

This looks really good! I would say some post processing to match the phones camera quality(more sharpness for example) would help it alot.

This looks great! If this were my groom I'd try the following to push it to the next level:

Does the tip of your hair width reduce to zero? If so try increasing it a touch. Your overall hair thickness might need to be bumped a tiny bit to give it a bit more definition.

The hair direction is also a bit too random, especially on the top part. You might need to increase your guide count to get a really nice spherical shape in the hairs if you're struggling to get good "coverage" otherwise.

Judging by the lack of white in the bottom half of the shape, I think the hairs need to be pointing a bit more to the camera at their roots, and then just the last little bit curves down a bit under gravity. I don't quite get the sense of a round ball of fur with just that single segment, like an orange segment, removed from around the face. Perhaps it is like that and is just being flattened a bit by the lighting/shading. (If so, maybe play with the Medulla radius in the Karma Fur shader?)

Lastly, in grooming, it all comes down to the clumping. Yours is really good, I think it could just do with a touch more strength to really get that definition.