I think that you do need to think about what you are hoping for regarding senior schools, and as you have noted you will do best if you are in a cohort of similar minded parents.
As a very broadbrush picture, the majority of RR pupils end up at the senior school, most Elmhurst pupils aim towards the grammar schools with Whitgift/Trinity as fallback, most CHS boys aim towards Whitgift/Trinity. CHS prefer the boys to stay until 13+, but some parents prefer to have the senior schools sorted earlier.
Given that CHS boys are aiming at indie schools, they obviously want to secure as many scholarships as possible. Given that these schools have a generous pot of scholarships funds for sport/music/art, it can be easier to get a boy up to the required standard in one of these areas with greater ease than getting them up to academic scholarship standard. The school has a busy fixture/concert list, so your son definitely won't be bored, and many children will also do sport/music outside of school.
RR prepare the children for their own exam but not for the grammar exams, and not so much for the competing schools. Of course you can still do so, and to be honest most children at these schools get some extra support if they are aiming for 11+.
You are right that co-ed schools and boys schools tend to have a different feel, with the latter being more "competitive" ime. And it is important to listen to your gut instincts as you visit the schools and your son has his taster days.
Across my children we have done CHS, the "junior school of co-ed senior school" route, as well as a co-ed prep school preparing for 11+, and of the three, we preferred the middle route - the stress of 11+ is minimised, the school continues a broad curriculum and prepares the children for senior school rather than for the test. That said I'm afraid that I have never seen RR's uni destination list as particularly inspiring, and the fees are a lot of money (if they don't help your dc secure a good uni place or other course of their choice).