Those are exactly what I have been thinking. One issue there is the table moves so far back that it covers the rear base but a pillar could go on a rear side. The unit is so tall i worry about having feeding issues if i mount it to the top but maybe its not a problem.
I keep coming back and tossing out the idea of the additional lead screw connected with a belt. I asked creality to send me the 3d printed parts files like the lead screw bearing support uses. The promised to send them monday. For your 500mm unit i think that would be almost mandatory. Do you know if the lead screws are the standard 2mm? I might just buy the $9 pair of lead screw bearings amazon has.