I'm no expert but I see the displasia in both right and left. Get a second opinion concerning the arthitis so that you don't question yourself down the line. There is a lot to consider. It sounds like you have done a great job ensuring that her hips were protected as a matter of course.
My guy is just shy of 8 and his has become painful. The difference between his X-rays at 2 and now broke my heart. If he were as active as yours, he probably would have shown signs of pain sooner but he isn't and he didn't so I just kept an eye on it. Also did the supplements, bone broth etc.
My thoughts, the opinions between Ortho will probably vary between right now or wait and keep an eye on it until mobility is affected, rate of arthritic change, signs of pain and which procedure is best all round.
my guys Ortho consult is in Sept. I'm not pushing for a sooner one because my gut is telling me not to. I started the Adequan shots. PT etc. it's helping but I'm pretty convinced that after his Ortho consult, he will be getting the THR.
Fwiw, I had a THR and revision on my right. My right was the better of my two but became horrifically painful. My left hip is still natural and going strong with no pain. My left was the worst one and looks horrible on X-ray and looking at it makes me wonder how the heck I can walk on it and it's been that way for a very long time.
I just wanted to offer some info through our hip experiences. And to validate that a bad hip isn't always painful, what is seen on film isn't always the total picture.
"If you can't see his soul when you look in his eyes, then you need a seeing-eye dog"