If I were to go back onto AD and switch from 15T to 35F, would this be good for the career or would I be way behind my peers?

I've always got the impression 35F was a fast track myself in the enlisted ranks (having been 35F myself).

Worst case, put in for WO after enough 35F time.

I guess the main question I have is that: Are you going to be able to keep your SSG or get dropped down?

35F is a great MOS. I myself reclassed to this MOS early I'm my career. There were lots in my class that were doing the same thing. The MOS had many jobs, but only 2 sides. You are combat, or you are strategic. I was combat my whole career. I deployed often, but learned so much so fast. You have to educate yourself on all the systems, they are your lifelines. It's a fairly difficult school, as the requirements to pass are pretty step, so you can't bs your way through it. But it's rewarding. Go for it... You won't regret it.