Sounds like you're off to a good start. I started with zanki and switched to light year. Even though Step 1 won't be scored for you, having a really good base for Step 2 will be key to doing well. If you haven't looked at the stats for NS I suggest doing so. I think it was the zoom meeting below that the Army PD says they want the brightest/best candidate.
I know people match after serving as FS, but it's to less competitive fields. I really recommend connecting to civilian PDs to see their thoughts on this. I also don't recommend putting all your eggs into one basket and making your life's focus NS. I'm just an MS3, but I had an idea of what specialties interested me before I started. I tried my best to keep an open mind during clerkships to see if other specialties interested me. Everything has a trade off and sometimes life's priorities change.
Here's a zoom meeting that was hosted a few weeks ago from Walter Reed NS.
Neurosurgery - Google Drive
"My thought was to try for mil match first, and if that doesn't work I'd enter civ match. If neither are fruitful then I would serve as a FS (which I wanted to do anyway) while continuing NS research on the side to build a more competitive application."
I think the Army takes 1 person a year for NS. If that isn't you, then you do an intern year. After that year you can reapply or its off for a FS tour. Unless the Army tells you that they are allowing civilian deferred/sponsored spots, you are not able to do a civilian match.
"However, I am confused about how payback/time served would work in this scenario. Would it be better to do 1 year FS and go for NS (would my pay-back be 6 years instead of 7)? Or should I complete my pay-back by doing FS for 4 years and then enter NS match?"
I'm assuming you're doing a 4 year HPSP plan. Once you are done with medical school you owe the Army 4 years. Intern year is a wash and does not add or subtract from that. Every year of FS you can subtract one of those 4 years. Say you do a 2 year FS tour, so you have 2 years left on you HPSP payback when you get accepted for mil NS. You'll probably have to redo intern year and then do PGY2-7. The HPSP years will count towards residency, but you start building up a new pay back because of the residency. By my fuzzy math, you will owe 6 years once you are done with NS residency.
The timeline would look something like this: intern year, years 2 & 3 FS, years 4-10 NS residency, years 11-16 mil NS attending.