Lose the ground to the main radio unit (i.e., the copper between the radio and the tuner). You don't need a separate ground on the radio...it's grounded thru the
power wire. You want to eliminate the possibility of ground loops insofar as practicable.
As someone said, take your power directly from the house
batteries. Not thru a panel or breaker. Put a 30A fuse in both the positive and the negative wires, located close to the battery. Use AWG6 battery cable for the run from the
batteries to near the radio, and connect to the shortened AWG10 power wires to the radio.
The KISS-SSB is a good solution, but there are others as well. A copper strip from the tuner ground lug to the nearest
bronze thru-hull works well. So, too, do radials (of which the KISS-SSB is just one way to do it, albeit a good and easy solution). There are others as well, depending on the layout of your boat.
The "100sq ft" is just an urban myth....long disproved by hams and professional SSB installers.
Even with the best SSB installation you may get some blinking of LEDs...it's induced RF into the circuitry. It may differ in intensity on various bands. Not much you can do about it except be sure you have a good working RF antenna/ground system, and that you have proper power to the transceiver.
Bill
WA6CCA