It might just take time. I would think there would be a fitness element to because I’m sure she’s been using herself differently since there was pain.
Did the vets recommend any strengthening stuff besides the lunging and riding? Thinking like carrot stretches, butt tucks, equicore or sure foot pads? I wonder if any of that might be useful in a gentle way to help rehab. I’m saying this as someone who’s going through stifle stuff and now possible SI strain with my current horse. Rehabbing is tough and nerve wracking!
I think it sounds like you are on the right track though. Fingers crossed!