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I'm willing to bet you it's the spiral couplers, I chased my tail with them until I used an oldham coupler of my own design, and then designed a magnetic coupler which worked great, I never got around to publishing those designs, but there is a similar one here: https://www.thingiverse.com/thing:804873
The idea is no side forces get transferred to the Z-rod, and the Z rod will need bearings at both ends of course.
Some people have had luck with this:

Or go with the Z-belts, I made it that way for this Z-banding problem.

CNC Router stepper motor to leadscrew couple and housing