The information that you had here is incorrect. Secondary schools do not move children to their ‘correct’ cohort. To do so, they would have to prove it is in the child’s best interests to lose a year of education, which is highly unlikely to be the case. This is why the FB group I mentioned is helpful, since people have the correct info. Not everyone on there chooses delaying; some do half days or flexi schooling, and many, like you, are researching. MN is biased in its own way.
I will give you my experience, since you have asked for it. I have a summer born eldest and youngest.
The eldest was always bright and has never struggled academically. However, being the very youngest for their year (should have been born in the next year but was slightly prem), they were shyer and also found some things physically challenging on starting school. They have always been the very smallest child and eg slower to run etc. They have continued to flourish academically but they have struggled emotionally. Reception was fine but they would go to the book corner and nap (having dropped naps aged 18 months) and as school ramped up things got harder. Starting secondary was absolutely exhausting for them: keeping track of all the rules, homework and new social mores. I wish so much that child could have been allowed a delayed start to school.
My youngest is also a summer born. That child, having had older sibs, was more mature and physically able than my oldest child at the same age. I still decided to delay as I had seen by then the toll on my oldest child of starting as a late summer born, particularly in secondary school. This child started school so much more confident even after a year of extra play. Rather than hiding behind me on the school visit as they did when rising 4, they were asking the teachers questions, making eye contact and totally holding their own as a rising 5 on the school visit. They are one of the most popular children in their class and always have a little entourage around them. Incidentally, they were never bored at nursery! No one ever says this about 1 September borns, and tries to get them to start school early. Nursery was great and super supportive and my child has had so much fun playing and learning there this year. It is a forest nursery so lots of practical and outdoor focus. But any nursery that is boring kids is not doing its job correctly. Incidentally the nursery of my older child merely said ‘they’ll cope,’ as though that was the bar that we should have for our kids. They did cope, but barely.
I will also say I was a late summer born myself and always felt kind of pleased with myself growing up that I was one of the brightest despite being the youngest. But I did have issues I wonder if I wouldn’t have had if I had been the oldest rather than the youngest. Eg lack of confidence in friendships; often being walked over by older kids in the year. (This persisted till uni, particularly as I didn’t take a gap year and many of my peers did so lots were 1-2 years older. Summer borns are also significantly and consistently underrepresented in my very academic and competitive degree course). Some of my issues may be personality but who knows. Professionally, I have been successful and done things early but I am not sure the value of this now I am officially middle aged. I certainly can’t get that extra year of play now!
The most important advice I could give is probably to look at the evidence around summer borns. There is a reason why the legislation was introduced.
But I am sure for some kids it might not be the best option. You have to decide what is right for your child. No one else can weigh the options. As well as looking objectively at the evidence, get a range of views. But bear I mind that those who decided to do it will almost always want to justify their actions, and those who decided against it will also do the same.
good luck with your decision.