I was in your shoes a few years ago. It was a dream of mine to work at a FAANG and when I got there, it was super disappointing for a lot of the same reasons (IQ comment aside). I consistently got good/great performance reviews without working nights/weekends and I could've kept coasting for a long time but I would've been miserable. I left and I'm back in the startup world now, which has its own problems but I'm happier.
The one thing I wasn't able to get from the post is what it is you actually want out of your job. It sounds like working with smart people is one component. You mentioned an efficient toolset. Is that it? I doubt it. It's probably worth taking the time to sit and figure that out before you move onto your next thing.
As software developer we're in a unique position of choice, and IMO we should enjoy that while it lasts. It's not guaranteed to be like this forever. Life, and your career, is just way too short to spend in a place that you know is wrong.
Do you think your FAANG employer would have a second thought about letting YOU down? Doubtful.
The first rule of survival is to take care of yourself so you can help others. In this case, self-care likely means leaving the job.
What would you advise a friend to do in this situation? Are you, as a person, willing to care for yourself the way you might care for a friend?
But a sense of obligation is not a good reason. You owe the company and the team nothing, your employment is fundamentally an at-will business deal, anybody who tells you that you're all "friends" or "family" is very confused or downright lying.
There's a pretty good chance the rest of your team has almost no emotional investment in the "success" of the team and couldn't care less if you leave.
There's nothing wrong with leaving a broken work situation.
One final thought is that I know I can quit - that isn't the question. I could make more money (with less stability) consulting, or find a middle ground at a Series C+ company.
What I probably should have said in my OP is that quitting feels wrong. I've never quit a position after three months and this is honestly the first time everything is telling that quitting is the right decision. As dysfunctional as this team is, quitting would feel like letting them down, and it just isn't something I've done before.
But again, these comments have given me perspective and will make me give this more thought.