Progress report:
All of the hot wires for the 2 main lines are labelled and connected to the MTH Terminal Blocks powered by one Z4000.
I have also disconnected and removed all of the ground wires for the 2 main lines except for the ground drop wires from the lock-ons. My next big project is to install the common bus wire the entire length of the layout and place terminal strips at 7 or 8 locations and connect the ground drop wires to those terminal strips (each with a bare 18 gauge copper wire threaded through the lower row of screws.
I now have substantially less wire underneath my layout. Instead of looking like a huge rat's nest/ bowl of spaghetti, it now looks like a medium sized one. LOL.
I have not yet started re-wiring the wires for the 9 sidings powered by the 2nd Z4000.