I never considered the starter battery voltage dropping as attributing to the contact resistance inside solenoids, but it does make a lot of sense.
I've got a couple of older solenoids and can feed them different trigger voltages and see how much amperage and total wattage the electromagnet requires to hold contacts closed at 10.0v vs 12.2v, but don't see myself measuring resistance across them under load, at those different voltages.
I've always sought a trigger circuit which was not live until after the engine started, next best was one not live during engine cranking, but would be on with the ignition.
I never wanted any delicate electronics hooked to house battery to get cumulatively damaged by the voltage spike when the starter motor is disengaged. but I've been effectively been doing that for 5 years as my house battery and starting battery are now one in the same.
I once installed a user provided solenoid, and that thing got excessively hot quickly, passing just 30 amps. I realized much later it was likely a latching solenoid that needed a momentary burst of 12vdc to latch or unlatch, not continuous 12v to power an electromagnet.
