With your dog's dominant personality, I think adding another more submissive dog would be great for you guys.
I am a big believer in dogs as pack animals, like birds are flock animals. We have a pack here at my house and they are so fulfilled as dogs, you can just see it. I love it.
And as for the odd myth that if you get two dogs, they will bond more with each other than with you? That's nonsense. Even with this great pack of dogs I have here, I am still the most important thing in their world, and each dog still needs individual attention and love from me every day. I am like their queen, lol. It's good for the ego!
Good luck in your decision!!!! I recommend a rescue!