nd. I did this because it says in the manual to avoid having a common ground with a welder. And I do use one quite often. I did not connect the shielding to anything. I.ve only tried yesterday to connect one end of the shielding at the vfd end to see if it makes any difference, and it did not, and also disconecting the spindle ground from the vfd did not do anything (as by default there is not a 4th wire available). No shielding it connected at any end for the steppers or the limit switches. The only non shielded wires are the mains for everything, pc, sources, drivers, breakout board and vfd.
All the wires go togheter in a cable drag chain.
It is important to connect all the shields to a star point at the control box. Including grounding the router frame.

Only ground one end of the shields. Also make sure the various mounting plates are also grounded to the star point.

With a gantry that wide I think you will need two screws to stop it racking this can be done with one motor and a long belts or two motors one each side.