voltage drop measurements in run

I've recommended this several times. One way around this is to lessen the load on the circuit. One way to do that is to feed a bosch style relay from the starter relay "big stud." Electrically cut the "run" line after it enters the engine bay but before it branches off to loads. Use the firewall end of the wire to trigger the relay, and wire the engine bay "loads" end to the switched contact of the relay

Pay attention to coil polarity, as most auto relays have a built in spike diode