" We like it but think for some reason its not as realistic as it can be "

It is obvious that it is a veneer and that you are not changing, As far as darkening the mortar it will be lipstick for which we have no idea if it will work as we have no context as to what the room is and how it will be decorated or used.

I’d probably go the opposite direction and look into a German schmear treatment to make it look more like actual old brick.

The reason it is not as realistic as it could be is because it is fake. A fake brick wall is just going to look fake. If you do anything to it, use the spare veneers as testers to try your new application.

My sister brought a house with fake veneer red brick as a backsplash in the kitchen. It was very ugly. She didn't have a budget to tear it out but she really improved the look by painting it a gloss off white, used a oil based primer after scrubbing to get it as clean as possible. She used gloss since it's easier to clean in the kitchen. It was a 100% improvement. It's not perfect but it's at least neutralized and easier to clean.

To tell you the truth I would live with it for a while as is before making a major change. It doesn't look bad in the picture. At least it doesn't give the immediate "I'm fake" vibe. Maybe only when closely scrutinized does it look fake.