Welcome to the forum. 
I don’t think anyone can give you a clear cut answer since everyone plays differently. All the game areas scale to a great extent, but I think either way would mean you might be a bit overlevelled. Both AoM and FG upped the difficulty a bit from base game and also if you’ve been playing Vet most of the time that has increased mobs/hero spawns plus an extra 10% XP on top. So really the choice is yours to either go FG on Normal or skip it and stay in Elite. Also bear in mind there’s nothing to stop you from leaving FG in Normal and going back into Elite main game if you want to. The expansion is entirely optional as is AoM. You have free choice of when or if you do them.
To get out of the FG faction rep situation you can quickly dive into FG by talking to the Emissary - he appears at all the major hubs - and then going through the portal. Do his first quest and then choose which faction you want to ally with. This won’t make you hostile with the other two; unlike the Kymon/Order situation you earn rep with all 3 no matter who you choose to ally with. However, I’m not sure you earn much rep for them in the areas outside of FG.
Edit: Forgot to add if you do the start of FG that’ll allow you to access the movement runes which are placed on medals. You might want to try them if you like that sort of thing.
They sell basic ones, but you can find blueprints for others as you level up and progress through the difficulties.