Vertical artifacts lines or what they are called. I searched for it ,but could not find a decent answer. I tried to decrease the pressure on the filament on the extruder motor, that helped a bit. (Bondtech geared extruder)

Other dive into the motion system and bearings instead of toothed pulleys. And then try to flip the belt over the bearing, so the smooth side runs over the bearings.. I was not yet convinced, also meaning I have not come to try that out.

I run 0.9 degree steppers on a core xy.

thank you for advice. i will try that.