Hi OP. I've had a lot of experience with Rosebery and on the whole it's been excellent. On the rare occasions that we've had a problem, the school have worked with us to rectify it. Of my two DDs, one stayed on to do her A-levels there and the other one is planning to, so I think that says a lot about how they see their school (I believe the majority of Year 11s stay on for sixth form and the number is rising year on year). I'd be lying if I said there was no bitchiness at all but I don't believe it's any worse at Rosebery than at other schools (I myself went to a mixed secondary school and definitely encountered bitchiness and bullying there, so I don't think it's just a girls' school phenomenon) and my impression is that the girls are generally supportive of each other. Also, because it's a large school, there are all sorts of girls there so there's lots of choice for friendships and your DD will almost certainly find her tribe.
Academically, it's an outstanding school with fantastic, dedicated teachers that really go the extra mile to support the girls. They have lots of initiaves to help all girls reach their potential. They run a wide variety of trips and experiences (everything from theatre trips and PGL to ski trips and geography field trips to Iceland) and there are clubs and societies every lunchtime too. The school is strict on mobile phone use, having a "blackout" system for the younger kids which blocks access to phone cameras, social media etc during the school day. They really understand girls, and have quiet rooms where girls who find the hurly-burly of the quad a bit much, can go to. There's a lot of focus on wellbeing and mental health.
Anecdotally, I know of a few girls who have struggled there, mainly those with ASD. It is a large school, and can be overwhelming for some (although I also know of other girls with ASD who have thrived at Rosebery!).
There are mixed schools in Epsom, such as Blenheim or Epsom and Ewell, and these are also good schools (although I don't have any personal experience of them) but I think the presence of boys sometimes brings its own drawbacks (as well as advantages). When would your DD be starting secondary school? Have you been on any open days yet? These can be a good way to meet current pupils and ask them directly about anything that your concerned about.