@Karma1387 as a parent and a teacher in a grammar school area, I’ll answer your questions and also add some advice/opinions. I’ve taught in secondary, primary, private tutoring, sixth form, and SEN in my county.
Grammar schools create an environment where the academic kids with engaged families and the resources to succeed (not necessarily financial, but cultural and emotional resources around attitude to education, having a household with wide interests and a family that discusses different topics are supportive) are generally separated from the other kids- those who are less academic and those whose families haven’t engaged with the system enough to put them in for the test. Those kids go to the “comprehensive” schools, which end up anything but, and tend to have above-average intake of free school meals and children with My Plan, My Plan + and EHCPs. As a result those schools have to do more with less.
Children who are coached through the 11+ often struggle at grammar school.
Grammars in my county don’t have catchment areas and take children from several counties away. It’s done on a ranking system and even if you get above the pass mark there isn’t any guarantee of a place, it depends where you fall in the list and how many people above you choose the school you want.
Non-selective counties where there is one school in a town tend to take everyone so will have a more balanced intake and offer a more truly “comprehensive” experience.
Grammar schools best serve academic children who are quite resilient. One local grammar to us has a terrible reputation for eating disorders and anxiety, another boys’ school has a terrible reputation for toxic masculinity and bullying, and anxiety. Like any school, there will be good and bad. And every child will have a best-fit school whether that’s affected by the journey they have, the luck of the classmates, their interests and aptitudes, or some other factor.
Primaries are different, their nurture and socialisation of the children are more important than the lessons, really, as you can scaffold your children’s learning if you think the school’s provision is lacking. Secondary can be like that too, but arguably it’s harder to compensate for a poor secondary experience.
Given all you have said, I would strongly urge you to take your time to find a place you want to live in for itself, and not worry about the schools there. A school that’s good now might be terrible in a couple of years. A terrible one could be turned around and be brilliant. Your kids are so young and you’re trying to future-proof your lives, but you can’t. You don’t know what sort of children they’ll be or what they’ll need.
If anything I might gently suggest you look for decent SEN provision- because whether or not your children need it, a county that supports its most vulnerable children and families well is perhaps more likely to have supportive, attuned, and effective education more generally.