Glad it’s alive
Dropping the spring-length add-on is fine if the margin already matches what you see. That term only matters when the thing you’re clamping and the camera are clearly offset; your setup can land without it.
The +Z jitter is very likely the SpringArm colliding with yourBounds_Colliders[3]. Pivot hits the wall with MoveAndSlide, the arm ray also hits the same shape and keeps shortening/lengthening → shake. Side walls don’t sit on that ray, so they look fine.
Likely fix is: put the camera-bound shapes on a layer the SpringArm does not mask. Only the pivot CharacterBody3D should collide with them. SpringArm can keep masking real level geometry.
Also, Logic_Zoom changes Size but never calls Update_Bounds(), so margins go stale while zooming. Call Update_Bounds() when size changes (or at the end of the zoom).
For 90 degrees / 45 degrees rotation later, rotating the bound offsets with the camera (or rebuilding margins in camera-forward/right) is the right idea; circumscribed rectangle works, just rebuild after each snap.