Hmm, I would say you need to tighten up the the middle. The rigid middle z wasn’t always used, it should work just fine without it, it was more for milling aluminum. To tighten up the middle tighten all the bearing bolts that have plastic on both sides that should tighten it up significantly. Should be 8 of them. Try printing something a little simpler like a cube, at like 30mmx30mmx30mm. Problems show up a little better in a larger simple object.
That zip tie on the extruder bracket, is it causing the extruder to lean, it will limit your z travel as well? It needs to be perpendicular to the build plate and clamped in pretty well.
Not that it the cause but that has to be way more than 8 inches of z travel from your build plate to the first z bearing bracket is only 8″?