I haven't read the entire thread, but to share my experience, I deferred my bright son. He's June born.
When I say he's bright, I just assumed he would be fairly intelligent in the future because both his parents are relatively academic. However, age 4, my concern was that his speech was very poor (he was having speech therapy) and that he was socially and emotionally immature. This was my reason for deferring his entry into Reception. He was able to remain at his nursery (his keyworker had also deferred her own child so was very supportive) and he started YR aged 5 and 10 weeks.
I have never regretted that decision. He started school much more emotionally mature, was able to listen and focus, and was really ready for slightly more formal learning. It's caused us no problems at all, and his reception teacher went from slight bafflement at our decision to suggesting it as an option to all the parents of the summer borns the following year! 😆
He's now year 5 and we are looking at secondary schools. We approached all 3 of our local ones last year and none had any problem with him remaining in his adopted cohort (it would never be in a child's best interests to skip a school year, so ignore that scaremongering!!). He'll even be able to take the 11+ with the rest of his cohort as it's age-adjusted anyway. He's not bored at school , he's in the top groups but not far and away the brightest. I think he's confident and thriving. He's not overly large or tall, he's not the fastest or strongest in his year group, it's never been an issue. Some children question how he's just turned 11 and is in Y5, but he just says 'I didn't start school until I was 5' and they all accept it.
It won't be the right course of action for every summer born child, but it was for us.