My dc have been at Catholic schools all the way through. In our area RC schools tend to be very over-subscribed, so almost all dc will be practising catholics, and there is an assumption that that is the background you are from iyswim.
There was quite a lot of faith-related stuff, including class prayer journals, prayers before and after lessons and annual retreats for each year group. The very few non-Catholics did tend to join in with religious activities (taking turns in reading at Mass for eg) - I think if you were going to take the 'opt-out' clause, you'd be better off elsewhere, but if you're happy to go along with the community aspects, then I'm sure you'd be made welcome regardless of your level of practice.
Having said that, I've not ever had the impression that it was oppressive or actively proselytising (though obviously if you were militantly anti-religion you'd sign up elsewhere). My ds was adamant he didn't want to make his Confirmation when he was in Y9, and he was in no way criticised or put under pressure for that decision. The ethos tends to be more along the lines of it being a 'faith journey', an invitation to a relationship rather than a forced one. The only compulsion will be that Dc will almost certainly have to do RE 'Catholic Christianity' syllabus at GCSE.
Sex-ed is (IME) not the strong point of most CAtholic schools, for fairly obvious reasons, but also because they will be aware that regardless of what the teachers think or would like to teach, some parents are very traditional and will be resistant to attempts to teach about eg. contraception. So you wouldn't choose most Catholic schools because of their forward-thinking approach to PSHE, but we've never encountered judgemental or dogmatic sex-ed teaching, more a bit woolly and idealistic.
In our experience Catholic schools tend to be very warm and supportive communities, with a powerful commitment to the pastoral needs of the individual - strict, yes, but done with a lot of warmth and humour rather than the chilly 'zero-tolerance' style of discipline that seems to be so popular in the Michaels-Gove-and-Wilshaw model. The only downside I would identify is that (probably because of the strong tradition of continuity) Catholic schools can sometimes be a bit slow to benefit from the opportunities of innovation and carry on doing things the way they've always been done rather than look outwards and embrace the new.
But for a child who is bright but in need of nurturing and pastoral sensitivity they can be just the ticket (my oldest dc was exactly like this, and blossomed in her small, traditional, supportive but academically challenging girls' school). We've had more academically rocky journeys with other dc, but still feel they benefited from the community and faith aspects of the schools. Our experience is of state schools, fwiw, but anecdotally I hear similar from people with dc in Catholic independent schools.