I LOOOOVE infrastructure stuff like this, because as Ron said, paying attention to the little things often leads to large rewards.
I believe I posted a YouTube vid a while back about proper wire joining and soldering techniques - and when I say proper, they meet Mil-Spec or higher. If they meet NASA's guidelines, surely they'll do wonders for everything and anything else. I'll have to go looking for that again. For that ring connector, crimping, soldering AND shrink wrapping is correct, as is the application of dielectric between mating surfaces and protectant on top of the mount point. (I like fluid film for that, and it's part of my maintenance routine to check and reapply as necessary)
Doubling up existing ground wires with similar gauge wiring is perfectly valid (I also had a website that showed what effecive wire guages end up as when you double and triple existing wiring)...but I'm a go big or go home guy, so if you're going to do it, don't be less than 2ga. double 0 is probably overkill for a car, but maybe not for a big diesel hauler or RV...or if you've got a kickin' stereo. Give those electrons a big, wide and smooth highway to travel on.
