They're very little to be nailing all of this to the summer delay, all this could change.
At 8, my August-born daughter in-year cohort was a year behind academically and struggled in just about every way. There were discussing of redoing a year or delaying entry to secondary.
At Y8, still in-year, she's middle ground academically, winning sports days and still faster than half the boys, and is more confident in school and socially than her February-born sister even with how hard all my kids found Y8 with puberty crashing into everything around them.
She's also more confident and likely to get better GCSEs than her September-born brother. The other end isn't all sunshine and roses when they're teens.
There is data around summer borns struggling more in some areas, but the same data shows issues for the oldest in year too, particularly as teens. Last I read, the oldest kids in year are statistically more likely to start 'adult' activities at a younger age because they're treated as more mature and make more friends with the year about them. I've seen this with my kids.
There are pros and cons to more cohort flexibility. Yeah, you have a wider gap between oldest and youngest - at the last school I went to, there was nearly 2 year difference between oldest and youngest. This can cause issues both for the oldest and the youngest children and more consideration is needed for things like sports and performances. However, it also means you can be more individualised to the child - when I started school as an August-born, I had to pass a school readiness test that involved me being observed while doing activities.
As one of youngest, yeah, it was noticeable in some areas, but it also played a big role in my finishing school because there were far more barriers to me just leaving compared to my siblings who were autumn and winter born. If I'd had to stay another year, I likely wouldn't because I'd have been able to leave without legal consequences.
The 'right' cohort is all about tradition rather than anything integral or better or more moral for children. It's okay to challenge tradition. England is really weird in it's history of rigidness with a cut off so close to starting so young. I don't think delaying is that great of a fix, but parents are bound by what the system will allow.