I've had a dabble with what you are proposing and find the '500 Rule' really is a rule of thumb. Your 50mm lens into 500 gives you a maximum exposure of 10 seconds and from my experience I'd go back to 5-7 seconds and take more subs.
I'd also suggest turning long exposure and high ISO noise reduction off 'in camera' and instead rely on your dedicated darks as it will eliminate the dark frames your camera automatically takes with each shot.
for DSS, a good freebie to get you started with stacking.
Experiment with different ISO and aperture settings till you find the ones that work for you.