I'm new to the Holy Grail method so I'll start asking dumb questions which is another way of saying "Why reinvent the wheel?" You guys have been there and done that so I'll trust your experiences to start with.
1. What's a reasonable shooting time duration starting point? Like start collecting images 1 hour before and 1 hour after after sunset?
2. Is a 6 second interval a good starting point?
2. Should I seriously consider an external battery power supply (like a 12 volt car battery) for my D750 and Android Smartphone?

Hi Tony,
those are basic, that are not easily explained in a few sentences. Especially the question about the right interval, and holy grail technique needs a deeper explaination. That's why I wrote a book in which I put 10 years of experience into 480 pages. It's available in English as EBook: https://lrtimelapse.com/buy/ebook/
I'd really recommend it to getting started. It will save you a lot of trial and error.
Best
Gunther

Thanks GW