Not radiographic progression, but some concrete evidence of active inflammation. My rheumatologists (both pediatric and adult) have done MRIs when I have had flares. For me, it's my SI joints and hips that are the problem, so those are the MRIs they do. If there is active inflammation, generally they changed my medications or increased their doses.
The problem is that without an MRI, it is really difficult to tell if there is active inflammation in someone's SI joints/spine. Especially since back pain can be a lot of things.
So most rheumatologists would want to see some evidence of a flare. Your CRP and ESR are currently normal, which is probably why your rheumatologist is not doing anything. I'm not saying that is right of her - a lot of people with AS (roughly 40%) have normal CRPs and ESRs. But I guess since initially your CRP was elevated before treatment, she's looking at that.
I would ask her for a neck MRI. You're right that x-rays will not show anything but radiographic progression. But an MRI will show active inflammation if it is there. Even soft tissue inflammation - enthesitis etc.
I think the official recommendations say that you need either an elevated CRP OR an MRI with inflammation of the SI joints to be diagnosed with non-radiographic axial SpA.
Similarly, I think you probably need one of the two plus symptoms for your doctor to take you seriously. It's possible that she thinks you're worried unnecessarily.But if an MRI shows inflammation, she has to do something and I bet she will.