No matter how ready you get there will be more questions or interviewers you can't pass. There is luck involved so it's just a numbers game. Keep trying, reflect on your mistakes and improve, and eventually you'll succeed.
I interviewed last year and got offers from 4 good non FANG companies in the Bay Area. I felt I was "ready" once I could see a new problem, try to see if I could see a pattern from a previous solved question, and make at least a decent attempt at solving. Don't worry if you're not able to fully solve it. The difference at the interview is that your interviewer will help you and try to get you to the right path(assuming you don't get an a hole). Like the previous comment mentioned, there will always be a question or an interviewer you wouldn't get past. It's all about numbers. I made it to Google on site 3 years ago, but failed in the phone interview last time LOL. Good luck!!