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Thank you for feedback. As I stated after printing Marvin, ToyREP is not good at printing round things, and I am still not sure why as you proved one of my theories with boudrate wrong (although I've experinced similar effect on other printers with NEMAs too, and SD card made circles more regular on them for sure).
Now It can be:
1, backslash in internal gearbox (that can be changed only with more precise motors)
2, backslash on belt (make them a bit tighter)
3, irregular shape of pulley (would be hard to get a better one without different 3D printer)
4, too high microstepping (try full step in X and Y axis to see a difference)
5, wrong decay on stepper driver (common problem on DRV8825 with 1/32 microstepping but hard to trace without oscilscope)
6, something wrong in Marlin firmware (hard to say, as almost all other firmwares have the same base except for Teacup which is quite hard to use for a newcomer)
Can you please let it print a bit higher to find out if this is regular or random (and also if you would not get layershift due to low current) and post me the result?
Thank you once again and I am sorry I can't help you right away.