OP I had two DDs go through one of the independent schools you mention as superselective. it was fantastic for one, she chose it over an even more selective school that was originally her first choice because she knew it was the school for her. However that was after she had had a chance to experience the atmosphere and had had a stimulating interview with the Head who she felt was interested in her as an individual and also injected some fun into the process. She is not especially confidant and was not one of the uber competitive overconfidant girls but she made a lovely friendship group and was definitely one of the brighter ones, though we had no way of knowing that before because she applied from the equivalent of a UK state school which did not prepare you or have experience of the schools.
My other DD also not especially confidant wanted to and did follow her sister though like you I always felt it probably wasn't right for her and though bright she would be better in a school that was more mixed, both academically and in terms of gender. It didn't work out, not because she didn't belong there academically, she was somewhere in the middle, but because she found herself in a very difficult cohort with a lot of disruptive, manipulative and exclusive behaviour and just did not have the skills to deal with it. She had lovely friends but they all felt under seige, constantly vulnerable to the attention seeking behaviour. She moved to another coed for sixth form, that you probably wouldn't put in the "superselective" category though like SHS still perfectly able to enable it's brightest pupils to get strings of A*s and to top universities, the one my instincts were telling me was "right" at 11. She found, as she put it, a "normal" and very encouraging atmosphere. She made a lot of friends, was happy and did if anything better academically and has gone off to university a much more confidant girl.
Unfortunately no parent can know whether her DD will encounter a cohort like that at any school (it can happen at any school but I think the competitive parenting that is an underlying cause is more likely to be an issue at the most selective schools). However I do think that if you and your DD have an instinct about which school is right then providing it enables it's brightest pupils to achieve their potential, and we are very well served with a wide choice of schools that do that around here, then whether they generally are able to select more of the brightest pupils, have a get better overall results and get put into a "superselective" category In the local context is probably less relevent. SHS in many other parts of the country would be regarded as superselective.
And totally agree with Needmoresleep on Tiffin. I have experience of using VR/NVR tests for recruitment and you pay for them to be constantly developed with unpredictable questions so that practise beyond a very few papers (less than 10) will not improve scores, that is the test of quality, that they are genuine tests of ability. The tutoring industry that has grown up around Tiffin to achieve alleged improvements in scores over years of tutoring with zero educational benefit (suggesting that understandably with the costs of testing 1000s they can't afford decent tests ) just backs up the evidence of their results that they were not selecting the brightest, though may do better with the greater evidence they will now have. My DD also hated it when visiting, felt the atmosphere was cold and slightly brutal and the Science labs were horrible. If you can afford to pay you certainly get more resources and facilities.
So basically do not be "very worried" , relax, look around the schools, choose the four you most like including one aspirational, one achievable and one back up. Go through the admissions process and then make the choice according to rational factors, like facilities, extra curricular, school journey, and whatever else matters to you but also don't be afraid to include that irrational judgement about which feels right, where you and your DD feel she will be most happy, you are best placed to make that instinctive judgement.