I'm nowhere near Solihull I'm afraid but there are a few things that jumped out at me in your post that I do have some experience of and that are universal.
The difficulty is that schools that have a reputation for being 'SEN friendly' end up becoming overwhelmed with DC with additional needs. In my area (London) there is a primary and a secondary nearby that are known for being SEN supportive - this means they now having something like a third of their intake requiring additional support. This is putting a huge strain on them. It also has a big impact on their exam results.
While it is true that mainstream, state-funded schools must try to support students with SEN, the reality is that it is becoming harder for them to do that.
It is also true that some schools (sadly) deliberately try to make themselves less SEN friendly to discourage parents from applying. These schools also sometimes try and 'manage out' students with SEN, to protect their results.
Do not judge a school solely on it's Ofsted (especially if it's several years old). You will need to visit them to get a feel for the school. It is often the 'Outstanding' schools that can be the least SEN-friendly (not all though). Also make sure you visit schools while the students are actually there. It's very easy to show off an empty school!
It is worth seeing if you can speak to the Sencos - although this can be difficult as they are so busy.
Otherwise, obviously ask questions during open days about how students with additional needs are supported: is there somewhere they can go during lunch? Can they differentiate homework? Can pupils have exit passes to leave during lessons etc? Their answers will tell you a lot.
In terms of applying for an EHCP - why do you feel it's 'too late'? I'm assuming your DS is going into Yr6? The whole application process should only take 20 weeks. It is true that many local authorities are taking longer (unlawfully) and you may need to appeal. Even so, there is no reason you can't get the ball rolling now. You don't say too much about what your DS's needs are or what he is like academically/behaviourally so it's hard to know if he would meet the threshold for an EHCP. The benchmark for an assessment is really quite low though.
There is some good information here:
https://www.ipsea.org.uk/Pages/Category/get-support
From what you say, it sounds like you feel his primary wouldn't be supportive of you applying for an EHCP assessment. If so, you can do it yourself. This is what I had to do after the Senco at my DS's school told me he would never get one so she wasn't prepared to do the paperwork. He did.
I completely understand your worries. I couldn't sleep for months before my DS started secondary school (he has ASD). He's Yr9 now and there have been ups and downs but it's gone so much better than I could ever have imagined.
Good luck!