Pass on the drag chain, just look at the diameters of all the cables and tubes you need to get through it and get whichever size will fit with some room to spare...

Most people use CY not YY cable since YY lacks shielding which helps protect other items from electromagnetic interference.

Generally 20mm plate is used since you don't gain much by using thicker plate and 10mm is certainly too thin. Also if you can get the bearings drawn in red further down, closer to the bed, that will reduce the overhang and make the axis stronger.