The only thing I can think of is take off the roller motor mounts and remove the whole center assembly. I know its a bummer but something wasn’t assembled right. Best to just re -do it.
Make sure the z axis is very straight, and the Stepper is mounted securely and square (I had one with a tilt and it messed with the threaded rod movement. Make sure the z rails are very parallel, put it together following the instructions, and check square and rail tension at every step. Make sure your nut lock is centered (it can move when tensioning the 5″ bolt). I know its a bummer but it is worth it to get it right. I just made a new stainless steel machine and it is so rigid it has to be assembled very accurately because it will not flex to allow any corrections like emt will.
My suggestion is roll the tension (center bearings on the longer bolts) with each adjustment. It is a very clear indication of rail contact. Try and get all the bearing evenly tensioned.
The seems to be a common theme for some, putt it together what seems to be perfectly, use it a few times and it is all out of whack. We can only assume things have seated or however you want to word it. The second time around it seems to hold its square/tension better. Of the three I have in use I just checked and one needs to be readjusted, the other 2 are holding up very well.