Nitrous plumbing A1A...??..

All set ups are different of course, I wouldn't necessarily worry that much of FP going below 5psi, I tried 4psi once from 6 on my rather rich cheater kit to see if ran faster, picked up a very small amount, I had a separate pump like you and my press was rock steady always, surprised yours goes down. Also short Fuel line and long N20 line is GOOD.