Recognise this issue? Can you now see why I want you to test without the bed on? I need to cancel the most likely cause (being bed thermostat).
I’d be very interested to see the photo of your cube without the bed on. And trust me. With this printer, that’s based on a shit load of experience.
Other potential cause is warps in Lead screws, although 2 features you may have noticed; the larger than necessary hole in the top of the 12mm rod holder for the lead screw as well as the leads being tied together with the 2020 aluminium bed extrusions are designed to negate this effect. (You have attached the 2020 extrusions to the bed mounts with screws and T slot nuts haven’t you) . In fact since implementing these features, the only place I’ve seen z banding to the degree you have experienced is with bed thermostats hence my interest in seeing a print with the bed off all together.