Worsening Cognitive impairment with Borderline CSF -- When to Retest?

In early 2022, when I was 58, my neurologist ordered several tests due to my having worsening short-term memory and other cognitive issues. A brain MRI showed "marked cortical atrophy", and a CSF test conducted by the Mayo Clinic showed abnormally low Abeta42 (679 pg/ml, normal range is above 1026 pg/ml), although my p-tau/Abeta42 ratio was within normal range, and cognitive testing was low enough to warrant a diagnosis of MCI. However, my PET scan was negative. My mother died of Alzheimer's, but her symptoms were helped by getting on Aricept and Namenda, so I asked my neurologist to put me on those drugs. I couldn't deal with Aricept, but I've been on Namenda 10 mg 2x per day since mid-2022.

In the last year, my cognition has gotten significantly worse, to the point that I'm unable to drive, work, and carry on normal conversations. I've had several cognitive assessments run by another doctor in the last year; the scores showed that my cognition was less than 50% of normal for someone my age (the last assessment showed 35% of normal).

At this point, should I ask my neurologist to re-run the MRI and CSF tests to see if there's been any progression that might explain my worsening cognitive decline? (I'm now 61.) When my PET scan came back negative in 2022, he seemed to rule out an Alzheimer's diagnosis completely, but could I have different results three years later? Don't get me wrong, I'm not begging for an early-onset Alzheimer's diagnosis, but there's got to be a reason why I'm declining so fast even with taking Namenda. Any recommendations or suggestions would be greatly appreciated!!