CS seems more consistent than ROF. Legal Cheek is also great too. Their ‘grading’ system can be helpful.I think Chambers Student and RollOnFriday are two underrated sources that go a long way at not only describing firm's practice but the trainee experience at each law firm. However, I think there are limits to what you can research. Not every major deal a law firm makes is on headlines and I've noticed that many firms don't document their pro bono work as much. A number of US firms only put up pro bono work done in the US, which while helpful is irrelevant to the UK practice. Lastly, there is only so much you can say to impress a firm about what practice areas you are interested in and talking about how good they are.
I agree with your last point. Hence, open days can be crucial. That and talking to the trainees.