With regards to negotiation, I know a friend who did this at a normal (not FANG) company and not transitioning from manager to IC. However, I tell this anecdote

With regards to negotiation, I know a friend who did this at a normal (not FANG) company and not transitioning from manager to IC. However, I tell this anecdote since you seem wary of your negotiating power. He was moving cross-country for family reasons. He said that the key to getting full remote approved was approaching his boss like “I’m moving out of state. I’d like to continue working here.” However, the important thing to note is he stated that the move was a done deal and implied that he was willing to walk away from this company and find a new job if he had to. You have to go all-in. Good luck!