RE:
Just confirming that replacing the hotend PTFE tube also worked for me. I had the same problem as the OP and replacing the PTFE resolved it. My best guess is that the inconsistent layers are caused by a random mix of over and underextrusion, caused by a faulty PTFE tube.
Here's what I think happened in my case, in more detail. First layer adhesion started to degrade for one of my PEI sheets so I (lazily) just decreased the first layer height (I should have just washed the sheet with soap or used a bit of sandpaper). I set the first layer height too low which caused filament to backup in the extruder and caused the extruder motor to click. When the filament got backed up in the hotend, it caused the nozzle end of the hotend PTFE tube to dilate slightly; I confirmed this when I pulled out the faulty PTFE tube - the internal chamber/portion was dilated at the nozzle end of the tube. This deformation in the PTFE tube caused the nozzle to both under and overextrude at random intervals (build up and then release). This mix of over and underextrusion caused the inconsistent layers or imprecision in the surface of the print. Notably, the inconsistent layer lines were the same between the X- and Y-axis - already a sign that the cause is likely not belt tension since this would typically affect just one axis.
So lessons learned for me: (1) Don't set the first layer height too short, it will damage the hotend PTFE tube and you'll have to replace it, (2) order several replacement PTFE tubes (e.g., hotend PTFE tube for MINI), they're only $1-2 each.