My DS has the same birthday as your DD. We didn’t defer him and I am not convinced this was the right decision, especially in light of the disruption caused by Covid - he was just settled in when the first lockdown started. Our reasons for not deferring were mainly logistical - our first choice school (an academy) “didn’t do” deferrals to reception for summer borns so if we’d pursued a deferral, it would have meant both leaving our childminder (who only covered that school for wraparound care) and finding a school which would support a deferral.
Our second choice school was very reluctant to even discuss it. As was our third choice school, actually, which we weren’t very keen on at all. We would have had a real fight on our hands and probably ended up with a non-local school and awkward school run.
He was also in school nursery the term we applied and if we’d deferred we wouldn’t have had the funding for it, apparently (free 15 hours), so he wouldn’t have been able to attend. So he would have been either at the childminder’s, bored without his usual sidekicks (who were moving onto school), or in a different childcare setting, and he doesn’t cope well with transitions. And I was also concerned that if we did defer him, we might have problems further down the line in terms of things like secondary school applications.
Hence we decided it would be better to ease him in with reception and continuity of wraparound care, and start him in school. He was academically ready and a similar size to the other kids but emotionally not ready at all, though two terms of school nursery before helped immensely.
I would say only defer if it’s to reception. DS found reception to year 1 to be a huge step up. He has really struggled. He would not have coped with direct entry to Year 1, where child-initiated play stops at the end of the first term. Reception is really important for settling into school.
I would also say that if you decide to go ahead, push to find out how the school intends to support your DD. My experience is that support is available for those who struggle academically and for those whose behaviour is disruptive. It does not seem to be there for those who struggle emotionally - DS is naturally very compliant and causes no trouble at school but found Year 1 very difficult and felt very under pressure. And school have been all talk about supporting him - but no support is actually being delivered.