The 5' cable should be fine, IIRC the SO3's shipped with a 6' cable. A 20' USB cable is asking for trouble.
If you are getting disconnects when powering off, are you turning the router off with the switch or using a power strip? If you are not using the switch on a power strip, try it and see if it helps. I ended up running my power thru an E-stop button for the router which helped.
I had one of the first 70 SO3's, so I am aware of all of the attempted fixes, I grounded my spindle mount to the Z plate, to the X-carriage, tagged both Y rails, back to the power supply, ferrite cores, capacitors, etc.
I had multiple ferrite cores on my power cable, now reduced to one. I am running a v2.3 board and have minimal disconnects. I will note that when I do experience one it is usually after a tool path has completed and I am moving the carriage or removing my stock from the table.
I have also noticed when the ambient air has less humidity, that is when I used to get more frequent disconnects.
I haven't removed any of my attempted fixes, it is running well now with an occasional disconnect, but I can typically recover using the homing and touchplate.