I would suggest that you have stiffness issues in your frame, slop in your drive belts and or vibration.
Try diagonal bracing, even if you cross brace with Dyneema or trimmer line. Any other damping will also tell you immediately that you have stiffness issues.
Check that there is no play in any of the bearings and drive belts
Your belts should be tight, play will result in backlash. Print a wall coming to it alternatey from the two sides, If it is not smooth, you have backlash.
The third photo looks like vibration occurring each time the (vertical?) axis changes. See what your code does at the junction this may give you a hint.
The first photo looks like you are not using enough chords when doing circles. IF you used Openscad, try the $fn at a higher value when generating the stl file.
4th photo - you have an uneven feed in Z, something is sticking
The long drive shafts on the axes may be flexing when under tension from the drive belts at both ends. This will cause vibration. You may need to support the pulleys more rigidly. Yes, you will be over constrained unless you deal with misalignment of multiple bearings. It may pay to use dual bearings on the pulleys and thinner drive shafts that can fles over their length and take up misalignment. Also try self aligning bearings.
Photo 6 could be vibration. Try reducing current on your steppers. If they are set too high, they will be noisy and will step with a motion that over accelerates, then stops on each step. This induces vibration.
If you are using allthread instead of lead screws on Z, try using couplers instead of nuts to drive Z. Couplers, being longer will even out the poor quality thread on what is essentially a construction material and not a precision item.