Did you ever tighten the lower lead screw mount forcing the plastic piece up against the vertical the vslot. If so it may have bent lead screws. The gap there is the part most people seem to miss and cause binding. If you bind it or force it laterally it may bend them.
Unplug the Z motor and see how difficult it is to send it up and down by hand. You should be able to move it very easily.
Yeah that idea to unplug the motor and try it manually is the best one. Could be that your frame is not square- that happened to me until I realized that I had to add a screw through every hole in the extrusion- in particular the screws that go into the threaded holes. Make sure all parts are tightly fastened.
There’s a lot of stacked tolerances on the lead screws so everything has to be pretty precise. I had to do this my first time around on the upper supports to get them perfectly aligned
I kept the delrinnuts “flying”, on the aluminium- L-frame, with the two bolts of the Delrin Nut just loose attached ; the bed is heavy enough for that, and no z-wobbling