RE: Ringing/ Ghosting pattern all over my prints :/
I also have done the same hardware changes/test as you - with the same conclusions.
Try to test accelerations from 100mm/s2, in my case that helps to achieve supermicro orange peel structure / no structure (depends on shape) which could be compared with bambu quality.
Although I use 1.0 nozzle and 20-30mm/s speeds, so steppers run really quiet + huge layer volumes damps some vibrstions (in my theory).
Greasing the linear guides makes the printer quieter, but does not help to reduce the VFAs.Replacing the belts doesn't really help either.I have also tested various pulley models without success.
During my research I came across an interesting website. https://prorifi3d.com/pages/technical-details The VFA problem seems to be a fairly old problem in 3D printing. After many attempts, my observations match prorifi3d's theory. The main cause is most likely the motor resonances and an irregular force distribution of the stepper motors. Since the VFAs change with the motor speed, it may help to optimize the speed of the outer lines. However, this only helps to a limited extent with a printer like the XL, because depending on the printing angle, one motor turns faster and one slower, and the VFAs reappear. I have just ordered a replacement motor from Prusa for further testing. This now appears to be a new model. It is significantly quieter, but also produces VFAs, albeit with a different resonance behavior than the old motor. The Orca-Slicer VFA test shows a clearly different pattern with the other motor model, but unfortunately not optimal results either. I'm now pretty sure where I need to look for a solution. However, motor optimization should be a big challenge.
Maybe also interesting for further ideas to minimize the VFAs: https://www.automate.org/motion-control/tech-papers/solutions-to-reduce-stepper-motor-resonance
I'll try to print the motor mounts out of hard TPU. Maybe that will help to reduce the resonances.