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.