@droftarts
I thank you for the very detailed answer that took up a lot of your time for me.
Let's go step by step:
Cases like this, unfortunately, are dealt with by the exclusion technique. That is, you have to check the many possible causes in a way that excludes them one by one and then examine another possible cause.
The mechanical part is the first to be checked, and from the checks done on the pulleys (4 of 8 were fitted new even though they were perfect for me) it did not produce any changes on the prints. The other four that were not replaced before were disassembled and tested by fixing them to a vise and putting them in tension with a belt and they kept swearing smooth so I think the pulleys are all ok.
The belt was replaced. Upon disassembly the old one was not in good mechanical condition even though they looked ok to the eye. The old ones were hard to the touch, almost wooden, and if they hung down they did not stay taut but tended to curve in places.
The new one, identical in make and model to the old one seems a bit too rubbery to me and I am waiting to receive new Gates brand 9 mm belts.
So, I would like to try Gates belts before venturing into the driver talk, which I study in the meantime.
In the meantime of all this, I also tried using Orca Slicer instead of Prusa Slicer and found no difference.
Instead, I found a VFA test inside Orca Slicer setting it with speeds from 40 to 200 mm/sec and got a print where the difference of the vertical lines, even at 200mm/sec was still visible though slightly less. I mean that in the face of a very large difference in speed the lines varied very little but they were present so now I have a doubt if indeed the driver speech is the cause. That said, considering that the common and unvaried part are the non-Gates belts (even if replaced) perhaps I'd better try these before thinking about anything else.
FYI:
My M569 P0.0 response:
M569 P0.0
Drive 0 runs forwards, active low enable, timing fast, mode spreadCycle, ccr 0x101b4, toff 4, tblank 2, hstart/hend/hdec 3/3/0, pos 0
Between yours and mine I notice the difference mode stealthChop / mode spreadCycle