@prismo I just did some more checks. If I adjust the z-steps to paper height I can put it out of alignment by pushing the right side of the gantry arm up or down. There obviously isn't perfect synchronization between the left side (where the drive screw is) and the right side (where it's just pulled along by the rest of the gantry).

I'm gonna play around with the eccentric nut and some grease to see if I can get it consistent and report back.

I just checked the right side of gantry arm. I can move it a little if I push kind of hard but there's no "play" in it. I don't think I could tighten that down any more without stressing the motor. I wish I had some reliable way to measure what's actually moving but the only movement I can detect is the z-offset drift.