So here's my attempt to help you identify the issues with VNAV still. I see there's an experimental download available, but I'm not sure where the "experimental" channel is? Can someone link there?
Airbus: A320 CFM
ERIXU UN860 ETAMO UN855 PPG UP84 ALBER
STAR: ALBER1S into RWY 25L
At CLE there's an altitude restriction on the star that says between 10K and 7K. After passing CLE, the plane did a dive upwards of 7,000 ft per minute to get to the next restriction (per screenshot) which I believe was the IAF at 2300. That's a HUGE nose dive, and this was even after I started regaining control, so the descent rate was actually worse. As you can see the magenta dot is at the bottom, and despite it chasing the magenta dot, I would've assumed that descent rate was far outside of the envelope for the plane to even do this automatically on its own. Additionally, notice the complete disregard for speed restrictions under 10,000 ft.