If you would like near perfect Z axis IMHO some Very important info:

1. As said above keep the bit tip inside the gantry legs bearings step spread, side view

2. Make the design so that the rails are mounted on the moving z plate and the bearing blocks are fixed. Opposite to what you have done, not that what you have done is not valid for small Z travel.

3. Idealy bearing block spread on Z and on the bearings that move the Z left right / X/ will for same squares, or at least same rectangles, and ideally would coincide exactly on top.

4. A must for my designs is that the spindle is mounted so/ 2 brackets/ that it fortifies the front bearing plate in all positions, including fully extended / especially fully extended/

5. Use square supported rails on Z at least, best Z and X at least. Even better on all machine. 20 size.



PS. Square supported rails don't care how they are mounted. Not so with the rails you have chosen. Ideally on gantry top and bottom, left and right on Z. Your drawing uses them in their weakest positions.