I perhaps should add that things seem to be improving, slowly. But I would say that is simply because of increased age/maturity. Kind of, despite school rather than thanks to school.
In my DS case also perhaps one problem is that he behaves super well at school (he is very anxious about being told off), he was recognised as being 'very smart' within weeks of starting, he is eloquent and has a great vocabulary. Although he is still in 3-4y clothes, and the year is older than average, there are a few kids noticeably shorter than him. Also he is physically fit, can run really fast, has great balance etc. So he doesn't appear like the class 'baby' and it is very easy to kind of forget about him as being young-in-year. But under all his ability to read and add and express himself and run, emotionally he was still that kid whose 4th birthday was less than two weeks ago when he started school. I do believe our school's pastoral care is reasonably good but perhaps DS' needs got a bit overlooked due to the way he presents.
Regarding missing out on friendships, well DS started forming a friendship with another 'loner' towards the end of the first term. Apart from that he has had one playdate. His best friends remain some kids he knows outside of school. I think for many children, age 4 is not really a time when they are keenly interested in 'friends' and I honestly do not think he would have missed out in terms of friendships by starting later/part time.
Regarding 'resources for settling in' well the first half term was totally crazy. There were 29 kids to be settled in and despite the teacher's and TAs' best efforts, many children felt very lost initially; up to October half term the majority of the children (including 5yos and hardened nursery attenders) were still crying at drop off. In the process of getting to know each other, things at times turned nasty with bullying behaviours (excluding, holding down and spitting in face, belittling, denigrating, intimidating, ... all this was totally foreign to DS and far from a 'gentle easing in to school', felt like brutally being thrown into the deep end). Things have improved a lot since then but I would not have been sad to have missed all that initial upheaval! In such an environment, it is very easy for the needs of a quiet, conforming child to be overlooked.
In contrast, one lone new kid has just joined, having deferred until now. He is met by a known person at the gate everyday, and has a buddy allocated, and has an adult looking out for him at all times. Yes there are less overall resources but the resources are not stretched between 30 kids either. And those other 29 kids are a lot calmer and not fighting out social hierarchies. All the other 29 kids know their way around school now and so it is easy for the new kid to learn how things work simply by following their example.
In our case it was about weighing our DS' needs with the needs of his new sibling, and there really was not much choice about it - we needed him to go to school, full time. But had I known just how it would turn out, I might have tried harder to find alternatives.
What contributed to swaying us was that we were told again and again how DS would clearly be fine at school and was so 'ready' and 'needed' to be at school - because he is academically able. But I think that is not a valid point. If a child is doing well with their reading and numeracy etc. it actually indicates the opposite of 'needing' to go to school - clearly what you have been doing so far has been working really well! And academic ability says little about emotional maturity but might make a child APPEAR to be more emotionally mature than they are, hence leading to their emotional needs not being met. So what I'm saying is, try to not let your child's academic ability sway you towards (or against) deferring entry. IMHO it is not really about that at all.
Lastly, thanks to those who 'recognise' our experience; it is sanity-saving! With people telling me all the time 'but your DS is doing so well!' at times it feels like I am imagining things. Then I remind myself that no academic achievement can really make up for spending your days being unhappy. Not when you're 4 years old, and not when you're 40 either. IMHO.