Homebrew PCBs

Sweet.

I've been pondering CNC recently, and offer some thoughts...

Would it help to have two drive screws per axis, to avoid racking?  My
router lift works that way.  Put them on opposite sides of the load for
the Z axis.

Have you looked at the Harbor Freight 1/8" air-driven grinder, as a
spindle?  I'm wondering if high speed air spindles would have less
runout than, say, a dremel.

Your Y-axis has upper and lower guides, with the load cantilevered off
the side.  Have you considered supporting the Y-axis the same as the X
axis, with a sort of table to mount the load on?